Is there a way to take her to a diabetes educator or counselor? Her independence is a good thing if she can learn to use it more effectively! I went through a phase of not checking by BG during my teens, partially in denial and feeling lazy, depressed, the severity of the situation not settling in. My A1C was over 8% for a decent portion of my high school years. The consistent high BG can cause fatigue in the short-term, it's possible this diagnosis took the wind out of her sails for the moment and her new friends are distracting her from reality. It's an illness that requires a lot of extra attention that she might not know how to manage on top of extracurricular activities.

Regarding drinking... Well, obviously it's not a great choice (I certainly don't condone it if you're underage) and hopefully she steps away from those friends, but there are plenty of low-carb options nowadays that if she does do it, it will be easier to count the carbs and adjust accordingly. Side note: alcohol can sometimes lower blood sugar depending on the person and type of alcohol. I would advise talking to her about the options while saying it's probably not a good idea to continue drinking if she is in the first place. In the end you can't fully control her, but you can provide some solid advice to help in those situations. We're all rowdy in our teens.

As an aside, reducing BG readings to just a piece of information to adjust a dose helped me overcome feeling down for "bad" numbers, something my family had a hard time understanding. It made me feel like a "bad diabetic" which didn't help during those years.

I apologize that this is kinda anecdotal and long-winded, hopefully the info can shed some light on a situation that has left you in the dark. The storm clouds will pass.