it's not as simple as that. at first it starts out where you do it to make yourself feel better. but there's a point where it becomes an addiction. You NEED it. you can't get through a day without doing it, and eating makes it that much worse. you stop eating certain foods because you can't stand them coming back up. You become secretive, isolated, scared of people finding out at every turn and the HARDEST part is telling people you love about it. it's the judgement, the disappointed looks, the looks that you know are asking "what the fuck is wrong with you, how the hell could you do this to yourself" . you don't think that people with an ED believe that every single person they walk by in a day is judging them? . it's not something that you can just wake up one day and say "hey i'm going to STOP starving myself and make myself eat then puke it all up, then do it all over again". It's like any other disorder - take alcoholism - once an alcoholic always an alcoholic - you can be sober for 15 years and then there's one seemingly thing that could set it off again.
As for your girlfriend, you have to be patient. It probably took her a long time and A LOT of courage just to tell you. You can't expect her to see herself differently in a day. People do have good points - let her understand that you care for her and want her to be healthy - of course tell her she's beautiful but not excessively. The biggest thing is slowly getting her to accept herself and acknowledge that she has an ED and that she needs help. Try and get her to talk to her parents, or if not, at least a doctor or a counsellor. Most schools will have someone she can talk to privately. Once she gets to a dr. just be there for her. she'll need someone who's not out to judge her and "isn't against" her.