The best thing you can do is be there for your friend - she may be really unhappy inside and want to talk to someone/let it out. If she is to get better, I believe she has to want to do so. Definitely approach and tell an adult who you trust as its important that your friend is looked after by her family/friends and school. I have had an
ED myself and it was so hard talking about it at first so I can understand why your friend is denying a problem and just says she is on a diet. I am thinking of you and hope everything is ok. Let me know.