Does my friend have an eating disorder?! PLEASE HELP!-?!

I made a friend last summer at bible camp. It was her first year at camp and she came with her friend. We all discussed how we were nervous for high school, and we talked about a lot of things including food, and how her sister who graduate college not to long ago had just gotten over bulimia. We were sitting on the swings one night after services and a girl turned to my friend and goes "Why are you so fat?" My friends face went pale she snapped back with "Why are you so rude and inconsiderate?" The girl said some smart remark back and my friend walked away and so did me and her friend she came with. That night she spent awhile in the bathroom. I think she may have been crying. But the next day she was kinda hesitant to eat breakfast but after a little bit of reassuring her she ate. I've kept in contact with her since camp, so today we have a Youth thing and she came to it. I saw her and she is like a skeleton now! Today at breakfast I noticed she barely ate anything a bite of her cereal and about 1/3 of her naked juice. I noticed when she finished she kinda poked around at her cereal and watched all of us eat. She was at least 155 at camp and she is about 5"6 or taller. Now she has to weigh anywhere around 100-90 pounds. I don't know what to do!