My best friend just confessed to me she has bulimia, what do I do?

Im going to be bluntly honest right now. If you love her and want the best for her, TELL AN ADULT. I know you dont want to go against her, and she will probably be mad at you. but whats worse? her being mad at you because you did what was best for her? or not telling anyone and have her slowly kill herself because you dont want to tell anyone?