My sister did the same thing at about 15 and believe me, in our family the women come with child bearing hips and discus throwing arms so skinny was definitely not her natural look.
I found accidently while raiding her wardrobe to retrive one of my tops and she had a list of goals stuck to the inside of the door, presuambly as a reminder. I was very concerned but neither of my sisters has been hit by the stupid stick (well, not too hard at least) so I just had a chat with her in the evening, pointed out that she had a lovely figure (they both do) and what made it even better was there was a definite butt, boobs and hips and this was actually very attractive, (in my opinion) and that although all three of us are disposed to putting weight on easily that not eating wouldn't stop that because your body clings on to fat even more then. We had a little chat about healthy eating rather than non eating and it seemed to do the trick.
I watched her quite closely for the next few days (including discreetly enlisting a couple of her friends to keep an eye on her at school) but she was fine. There's a lot of peer pressure on teenagers these days, both male and female, and what it doesn't take in to account are body types or healthy eating and exercise etc.... we live in a culture that demands a quick fix for everything including perfection. So while I don't necessarily think that screaming from the roof tops that this girl you know is in trouble, I do think someone in her family needs to know. I hope that like my sister it's a phase but if you're saying she's already very thin then I think someone in her family needs to be made aware. As they see her everyday they may not have noticed the change.