Anonymous wrote:OP -- I read what you said about what you have observed. Some advice.
First, do NOT bring up the weight fluctuations. She knows, and she's already thinking about them all the time. The most important thing you said is "I see you tormenting yourself and even when successful with your diets, you're not happy. Maybe you should see a professional to try to get at the underlying issues and who can help you with underlying strategies to cope. It makes me sad to see you unhappy."
You are NOT her dietician or exercise coach, or even her shrink. Many food issues come from a desire for control, and having someone else trying to control you just makes it worse. My DH was a saint when I went through a period of extremely disordered eating when we were dating, and he never said a thing because he knew I had to figure it out myself. The one thing he did do is realize that I was the most crazy when I was particularly starving, so he'd just say "hey, let's go out for lunch today. I feel like cheeseburgers". For some reason, if he said we should have cheeseburgers, I would eat one at the restaurant, and I would be a lot less crazy for a while because I actually had food in me. But he never said 'you're too thin, why are you doing that, just eat food, you have a disease, stop that, let me take care of this for you, etc". Because I would have doubled down.
I think this is good advice. Wait until she brings it up next time and try the "I see you tormenting yourself..." line. See what she says.
Also - would she be open to reading a book about habits together? I love Atomic Habits and Tiny Habits. Her late night snacking is a bad habit that she is capable of changing. It might spur her to make a change.
Ultimately, only she can make any changes. You just have to figure out the best way to support her.