I did that also. I couldn't completely give up food, because then my stomach would growl constantly and people would know that I hadn't been eating, so I would go for about 3 days without food, the most I could go without, and then I would eat alot for 1 day, and repeat the same cycle.
Thing is, once you convince yourself that you look fat, nobody can persuade you otherwise. The only person who can change your mind is yourself, and sometimes it doesn't even work then. The hardest part is when people won't believe you. For instance, I would tell people that I did have an eating disorder and they would say that I'm lying. People still do it. Also, when I would throw up, people would say I'm bulimic, and I would say I'm not, but again, they wouldn't believe me.
I know you want to help your friend, but only she can help herself. The only thing you can really do is let her know that you'll always be there for her, and if she tells you something about herself, don't judge her, laugh at her, or say she's lying. After you do that, it'll be almost impossible to gain her trust again. It sounds scary, I know, but it's all you can do. I hope that helps.
Also thank you for your concern. That means alot!