My experience with this problem is that you desperatley need to talk to your friend about it.
I found out not too long ago that my cousin is goin through the same thing as you, and i was devastated when she told me. But i'm so glad that she did because we could then start to do something about it. You shouldn't ever feel so down that you need to do something like that, especially as, from what you say in your post, you are an intelligent, pretty, and very slim person. My cousin has said that she feels relieved that somebody knows what she's going through, and although she sometimes still feels the urge to throw up after meals as a force of habit, it's easier for her to stop herself as she knows that somebody's watching her and constantly reassuring her that she doesn't need to.
We also went for a walk down to the beach the other day so I could show her what 'fat' really is. And at size six, that is something neither she, nor you are.