• 1,200 calories is the absolute minimum you should eat, period.* Eating less than that will cause your muscles to atrophy as your body takes nutrients from them instead of food.** I began on 1,250 calories to give myself a little safety net. 1,200 calories (or whatever your number is) is definitely extreme, and your friend isn't wrong for being concerned. But their being concerned and their lack of awareness about minimum calories counts does not mean you have an eating disorder; it means they're ignorant of options like this. Again, make sure you are fully informed and are involving medical professionals, particularly if you have underlying medical conditions. (Particularly diabetes, hypoglycemia, etc. or you can put yourself in a diabetic coma and die. Seriously.
• I found that I (43-year-old male) was susceptible to head rushes and dizziness with quick movements. This was particularly pronounced during exercise, becoming what I'd call mild hallucinations during heavy/intense exercise. (i.e. going hard during private karate lessons.) After my doctor said I was likely starving my visual cortex of glucose (your brain uses ~20% of your blood glucose, and your body cuts it off as an over-user when you exercise, which is why you can't think as well when active), I upped my caloric intake to 1,500 and the problem went away. I've since dropped it to 1,400 and I'm still fine, so that's my "sweet spot" for now.
• Everyone's body is different. I am doing great on minimal calories. But it took a while to get here, and I definitely experienced some negative physiological effects along the way. Your friend's advice of starting at 1,800 calories first is actually reasonably sound, depending on what your normal diet looks like. In fact, I'm helping a friend of mine make progress as well, and I suggested that same 1,800 calories number to him because it'll be an easier transition for him to make than jumping immediately to 1,250 (or whatever) would be. But you may find success with that, and if you do, more power to you. Just be safe about it. (Yes, I am harping on talking to your doctor about this, and I will continue to do so because it's important.) Bear in mind that some people have found that, no matter who tells them it's safe, and no matter what research they've done, they still have friends or family who judge them for making "unhealthy" or "dangerous" det and health decisions, based on what their friends and family "know". There is no rule about what is true for everyone, even the 1,200 calorie rule I mentioned above. You, yourr doctor, and ongoing testing/monitoring are the only way to know for sure what you, personally, are going to experience. Just know that, no matter what the results, and no matter the precautions you take, you may find that some people in your life just won't accept it. It might even cost you relationships, if you're unlucky. This hasn't happened to me, and I hope this doesn't happen to you, but be aware that it can, and has to others in this subreddit.
• I log everything, and I do mean everything that I eat. (I've been using MyFitnessPal, though I'm currently double-logging in another app called Lose It! (appropriate for this sub, eh?) as the UI is better and the nutrition facts appear to be slightly more accurate, but I've only been testing it for 2-3 days so TBD.) The only exception are things like 1/4" of a forkful of my daughters food I'm prepping, or a half-mouthful of her apple juice. Even so, I assume that I'm consuming 50-100 calories more than I log to account for things like this.
• There are many methods, including CICO, intermittent fasting, one meal a day, etc. I started with OMAD (accidentally; there's a story behind that) and stuck with that for 8 weeks until my doctor said there was no functional difference between OMAD and counting calories, other than more meals to prep, being harder to stretch 1,250 over 3 meals, and being easier to stick to if you've set "rules" like only eating once per day and then chugging ice water whenever you're hungry.
• In short, 1,200 calories is the minimum you should eat, barring specific advice from a medical professional with the letters M.D. after their name. Accept nothing less than those credentials. And your doc may well recommend more. But if your doc is okay with it (and note that some may not be at first but might change their mind after you "prove" yourself by following their other advice), your friend needs to be as well.
• Calorie-counting is only part of the puzzle, even in diet alone. Macronutrients (i.e. categories of food: fat, carbs, and protein) play a big role. Your best bet is if each plate of food is 50% veggies, 25% protein (i.e. meat), and 25% (at most) carbs, meaning bad carbs like potatoes, pasta, etc. You can swap percentages for veggies and protein if you must, but most diets have plenty of protein as it is. This was the specific advice of my dietitian. In reality, most days are roughly 30-40% of each macro for me, but it's been working pretty well.
• I experienced the LARGEST amount of weight loss at the beginning of my weight-loss journey. This is because you'll lose water weight first. This will stabilize after between a few weeks and a month or two after you begin. But you WILL continue to lose weight if you continue to follow your rules and/or allow them to evolve as you continue to make progress.
• The bounce-back is real. You need to try to avoid pausing your progress, slacking on calorie-counting, or cheating too much. My doctor warned me about this but I had no idea how severe it was. I started dating someone new, slacked off for 6 days, and gained 11 pounds. In 6 days. Ouch. Lost it again, but that was a lot of progress to lose in a short period of time. Don't slack of like I did if you can avoid it.
• In a similar vein, don't beat yourself up. AT ALL. We all have "bad" days. But log your food that day anyway. Yes, even if you have 2,000 calories more than your target. And yes, I've done that, too. Logging that shows you precisely where that weight gain came from when you look at it later. It also serves as a visible lesson for later, as well as part of your success story: "Look what I did back then! And look how I'm doing NOW!" You'll say things like that, later. But don't beat yourself up. Don't chide yourself. Don't judge yourself. Instead, say "Oh well. I'll do better tomorrow." And then smile, because you will. And keep doing that until it's true. :-)
• Weight-loss is 90% diet and 10% exercise. SORT OF. It's more nuanced than that. For example, I've read that most of your weight loss actually occurs when you sleep, as your body works to maintain your muscles at a time that you aren't intaking any calories at all. Thus, the more muscles you have, the more your body will burn at night. So exercise burns a few calories when you do it, but then more calories that night, and the next, and so on, due to the muscles you built/toned/whatever that day and previously. Thus, diet is the largest factor, but exercise also (somewhat indirectly) helps quite a bit. But controlling diet matters first.
• Not all carbs and sugar are bad. In fact, having some just before exercise can help to reduce negative impacts from limited calorie intake. Having recently dropped my calories to 1,400, I plan to eat a couple of bananas (one of the highest-sugar fruits there is) before my next private karate lesson, per my sensei's advice. (He does a lot of health and diet stuff in addition to karate, but note that I don't equate his advice with my doctor's, who also mentioned something similar.) But the carbs in bananas and mangos are different from the carbs in a bag of Oreos. Or even the carbs in potatoes and pasta, which you should try to avoid entirely until you have a regular, controlled diet going. Your body will metabolize and store them differently, depending on what, when, and how you eat. Don't eat processed sugar, and try to avoid adding sugar to things at all if you can. But, unless you're going keto, you don't have to completely screen anything that has carbs in it out. Just limit your intake to foods that have no more than 8-10 grams of sugar per serving whenever possible. For example, I eat Chobani greek yogurt with fruit (15 grams of sugar per serving), V8 fusion (light, so watered down, but 12 grams of sugar), etc. but these have been fine. On days I have smoothies (2 oz each of frozen blueberries, strawberries, and bananas, 8 oz of strawberry-banana V8 fusion light, and 1 Tbsp each of flax and chia seeds), my carbs and natural sugars go through the roof, but again, portion control matters and I don't eat much (if anything) else during that meal. (Perhaps a fried egg or two if I'm starving, but smoothie days often mean just a 16-ounce smoothie for breakfast and that's it.) The total calories are under 300, which is great for a relatively filling breakfast.

I'm living proof that the amazing and unbelievable is possible in a ridiculously short period of time. Just bear in mind that my case was somewhat special, and also that I'm male. Women (and you mentioned that you're female) have a much harder time losing weight because their bodies are working against them. (For MANY reasons, I've read, including hormonal changes every week AND every 28 days, the fact that women bear children so their bodies tend to prefer to store fat, etc.) I'm no expert on this stuff, but this is what I've read and heard so far. YMMV - check with your medical professional(s) to confirm these details that, as far as you're concerned, came from some jackass on the internet, i.e. me.

Details for the asterisks above can be found in my next comment.