God set me free from anorexic thinking. I had it from puberty on. It got so bad that I was in a museum once and found myself critically looking at a human skeleton (true story.)
It warps your thinking beyond belief.
Anyways, (I also remember seeing a documentry on tv of two aneroxic twins a couple of months before one of them died and thinking they could lose a few pounds.)
It's a mental condition.
A lot of women struggle with eating disorders. I was set free at an alter call. Just 'boom' and no more problem.
It never fully manifested with me so that I ended up in hospital or anything, but it could have. I think it was only Gods protection that it didn't. I was pretty messed up as a teenager.
Will add my prayers. God definatly can set your friend free.
PS. Eating disorders can have their roots in trauma or emotional issues. So sometimes seeing those healed can cause the overlying issues to dissolve. So in most people it's a process. But occasionally God will do a miracle for someone in a given area as well. Just instantly. (For example I had a friend with chronic fatigue syndrome who suffered from it for ten years. She went through a long journey with God healing all the causes of it (which were many, mostly emotionally and physical abuse issues) and when that was all dealt with he healed her instantly at a conference. I was sitting next to her at the time. She met/got together with her future husband that very night and married shortly afterwards. She also went into full time teaching a month or so later.
Sometimes healing is simple and sometimes it isn't. But God is always bigger. If you can support your friend in anything she wants to talk about and be there for her if/when she goes through anything painful, no matter how implausible or unrealistic it might sound to you, it could prove to be a big help.
God bless