My friend has bulimia?

Speaking personally, the simple fact that she told you means that she trusts you as a friend and truly wants your help to recover. You should be there for her to talk through what she's feeling, but unfortunately the best help you can give her is to get her some medical assistance. My suggestion would be to take her to a professional she could trust, perhaps a psychologist at a hospital that she would feel more comfortable with than the school nurse. She might resist it at first, and you may need to be really tough with her, and she will probably be pissed at you. In the end though, she'll appreciate you for the tough love and you'll feel better knowing you did all you possibly could to get her better.