hihihihi It's hard to understand from an outsider's point of view, exactly what is going through a person's mind when they starve themselves or throw up after eating. For your benefit I think you should probably do a bit of research into eating disorders, and the behavioural/psychological/physical effects it can have on a person. This will give you some knowledge as to what she may be experiencing, and it could also give you an idea of how to help her. Understanding the issue is usually a very good point to start. There are plenty of websites all around the Internet, and we even have a fantastic Eating Disorder FAQ located on the first page of this forum.
Encourage your girlfriend to talk to her mother. Her mother will be more equipped (mentally) to assist her daughter in recovering than I think you probably could. But that isn't to say you can't help. She probably needs you now more than ever. You need to tell her daily how beautiful she is, and encourage her to eat. Small things like chopped up fruit and vegetables, and make sure she drinks a lot of water. Encouragement is always a big thing for people suffering from an eating disorder, especially if it's encouragement from someone who means a lot to us.
As for vomiting, try being with her and occupying her time after she's eaten. Play a game with her, talk about something in-depth, or even play-wrestle. Something to keep her mind off vomiting and food will be good. I also suggest you get in touch with a doctor or therapist and ask them some questions. Or even (if possible) convince your girlfriend to see a therapist. It will help with her mentality of having an eating disorder, and breaking this habit.