I do not think you can stop it in its tracks, but you can flag it up as a matter of concern. Obviously only her parents can take action, and first you need to talk to them. Have they noticed that she is loosing weight and how little she is eating?
At 12 I think parents can still insist the child sees a doctor for a check-up, telling the doctor the child has lost weight and lost her appetite. (They shouldn't talk about eating disorders in front of the child) Loss of weight and appetite can also be symptoms of illness and while the doctor should pick up the hint at about a possible eating disorder, he will also undertake tests for any possible underlying medical problem. I fully understand your fears, but their could be other causes.
My 8 year old DGS is currently receiving medical help because he is underweight and anaemic. In his case the problem is partly because he is a wispy little thing with a small appetite, and has always been so, but because of that he picks up infections easily, when he is ill he stops eating because eating makes him feel sick, so he loses weight and is more susceptible to infection etc etc. He was taken to his GP and referred to a paediatrician. He has been given an iron tonic and is improving and eating better and is being monitored in case there is any other underlying cause.