There's a profound difference between worrying about one's weight in staying at a healthy weight, and anorexia.
I'm in my 50's, going through menopause, and can no longer eat whatever I want because it'll burn off.
I'm up 15# from my healthy weight of 135, I've put those 15# on in the last 10 years, 5# in the last year alone, and before I officially join the massive majority, I'd like to take a few of those pounds off. In other words, I'm seeing a trend towards overweight and I'm just not going there. So I've started watching what I eat, making the healthier choices, and passing up the junk food and desserts more often.
Anorexia, on the other hand, is a life threatening disorder, and you are RIGHT to be concerned about your girlfriend. It takes a serious commitment on her part to being healthy, and it may require hospitalization, both medical and psych. It is indeed slow suicide, and intervention may be necessary.
If I were you, I'd do more research into anorexia, it's treatment, and how to best support recovery. She's not just concerned about her weight, she's trying to erase herself.
No, no, no.