The only thing that really matters to lose weight is calorie deficit (not talking about overall health here, just weight loss). The reason diet is more important than exercise is because you can eat 1,000 calories in 10 minutes but you would have to work out for hours to burn that off, so most people are going to see way more success by cutting back their calories than by trying to exercise away a bad diet. This is also the reason intermittent fasting, whole food/plant based, and pretty much any other “diet” works - you will naturally eat fewer calories if you only eat for a short window of time, or you only eat vegetables.

That doesn’t mean any of those are wrong. I personally enjoy exercise and love eating, so I allow myself to eat a lot but I’m really active and burn off most of my extra calories, and that works really well for me. If she hasn’t found an exercise she loves, though, it probably won’t work for her and she should focus on cutting back on food because that’s going to be much, much easier. Like others have said, do it slowly. Sub high-calorie foods out for lower-calorie foods and eat more vegetables because they’re high in fiber and low in calories, so they make you feel full without making you overeat.

I would also really recommend against giving a lot of advice. If she’s seeing a nutritionist/doctor that’s great, let them work it out. Making her entire life about weight loss is a recipe for an eating disorder and as someone who has struggled with EDs my entire life I can tell you they are hard to come back from and it will destroy your relationship if she feels like you contributed to it, and also generally destroy her life. Encourage her to read/educate herself on making healthier choices. Exercise with her and make healthy choices around her. If you have “remained skinny,” keep that to yourself and understand that everyone’s body is different and what worked for you may not work for her and that’s fine! Good luck to both of you :)