Possible eating disorder?

It could be that she's just looking for attention, I'll admit some of the facts suggest that, but that's not necessarily a reason to ignore her (I know you didn't intent to). Sometimes people will go to scary measures to get noticed, particularly in their teens. Maybe get someone is high authority (teacher/parent) to have a serious talk with her and scare her with the dangers of anorexia. Or suggest that she goes to see a doctor. You're right to be worried and a good friend to want to do something about it...I'm afraid I'm no expert, but I've seen friends do this and it go a bit wrong. :/ Best of luck to you.