Re: My friend's problem
First, you would need to make sure that his weight and his vomiting are correlated. Alot of guys are skinny, that doesn't mean they're bulimic. And, throwing up (once?) doesn't necessarily mean it, either. Cafeteria food can do that to you, ya know?
If you observe this for a little while and it continues, then the only way to help him is to talk with him about it. Do this gently, ask him if he's ok, tell him you've been seeing this pattern. Tell him you'd like to help, and you want to go to the school guidance counsellor with him. If he refuses, or gets real defensive, then go yourself. If all this feels waaaay to intimidating, then AFTER you're really sure he's inducing vomiting, go to the school guidance counsellor yourself.
Doing the right thing isn't always easy.