I was in the same position as you a couple of years ago. The first time I visited her was so horrible, it was a proper hospital, rather than a clinic for eating disorders. I just tried to act normal, (she's a family friend, we had many holidays together as children and I've known her since birth!) so we just talked about memories and families etc. I asked how she was and she started talking about the anorexia of her own accord, not how she felt, but the facts, how much weight she'd put on, how much she needed to put on to be able to do more stuff etc. I'd say just try really hard to act normal.
On a brighter note this friend, while the mental issues are certainly not sorted, she has been out of hospital for over a year, and is doing well at school, and a great weight. So it can get better!