Hi J.! I don't know if I really have advice for you, but wanted to write to you because my senior year in high school I did the SAME thing. I'm 5'8" and I went from 145 pounds to 115 in matter of months. What did it for me was a couple of things. 1) It seemed that all the girls in high school were 5 feet tall and weighed less than 100 pounds. I wanted to be that cute little girl. I didn't see at the time that because I was a swimmer and I was tall that I looked just fine. 2) I was really good friends with a cross country runner, and one day we both stood on a scale at his house and he weighed a little bit less than me. I was mortified! Girls aren't supposed to weigh more than boys! And so, the weight loss began. I still ate, but when we'd have tacos for dinner I would have ONE, and I'd measure grapes into a 1/2 cup because I knew exactly how many calories were in it, and I stopped eating junk completely (I suppose that part was good!) I drank about a gallon of water every day - seriously - to keep my stomach full. Once I'd lost about 10 pounds I actually felt more and more motivated to lose more, because I thought I looked good. At one point I did start to feel a little obsessed and out of control, but no one ever approached me or said anything. I almost wished they would have. Not in a pushy way, but in a way that i knew they cared.
I left for college that Fall and began swimming in a Division IA league. My swimming was horrible (I had taken the whole Spring of my senior year off) and I attribute to the weight/muscle loss. That bugged me a lot. I think the real turning point for me was when we were in a dining hall after swim practice (where we had swam about 8000 yards) and the senior captain looked at my tray with its turkey sandwich (no mayo) and orange, and said, "Seriously?! Is that ALL you're going to eat?" It was almost like she gave me permission to eat again. I tried it - eating just a little bit more each day. I didn't gain weight and realized I could actually eat more than I had been. Well, swim season ended in February, and I gained 25 pounds from then until my parents came to get me in May. Obviously I didn't like that, but the crazy feelings of wanting to lose tons of weight were fading. I did fluctuate between 130 and 140 for several years, until I finally realized that my body just wants to be at 140 - it's almost like that is my set point. Does that make sense?
Anyway, I am 37 now, still 5'8" and 145 pounds. I do realize that I am one of the thinnest of my friends, however I still struggle with all of it. I exercise sometimes to the point of obsession and I think about food a lot (how much can I eat today, what will I be eating later?, etc.) It's hard having these thoughts all the time. I've never sought counseling, sometimes I think I should. I don't know.
So, your daughter may end up being like me. I guess my suggestion is make sure she you tell her you care. Maybe get her friends on board with you? I know having that senior talk to me like that (when I was a freshman) truly made an impression on me, maybe more so than if it had been my mom. Well, I hope she is OK, and I wish you well :)
K.