This is a tricky one. If your BFF has an eating disorder, it is serious and it is dangerous. Typically, when somebody is engaged in disordered eating behaviour, they are not open to listening to reason, and it is not easy to know how to help them. There are many people who have dealt with exactly this situation before with their friends and family and that have made great resources available to others that want to help. One of these organisations is called "Andrea's voice". It was started by Andrea's parents after Andrea, who had bulimia, died when she was just 19 years old. You can look them up at www.andreasvoice.org and read through the recommendations they make. There are many web based resources and also books that are listed that will give you and idea of where to start. It may also be a good idea to talk to a teacher or parents about your concerns. Eating disorders can be fatal. It's difficult to know what to do, but worth it to spend some time finding out!