My best friend might be developing an eating disorder im really worried. what should i do?

Talk to his parents: If they feel the same way, consult with psychiatrist.

Talk to her: You can talk to your friend about what you've noticed, and share your concerns. Before doing that, you can educate yourself about eating disorders -- national eating disorders association at http://www.Nationaleatingdisorders.Org/ has a wealth of information that could be useful. Review that site & you'll see that effective treatment is possible. Your friend may or may not be interested in this.

Early stages: You are a good friend for asking here. Like a lot of things, it is easier to treat an eating disorder in the early stages. Approach your friend from a perspective of compassion. She is suffering in ways that she isn't aware of. Can you tell her your concerns directly? If she isn't ready to hear them it is normal. Please give us more specifics about the situation. There is help. Good luck.