I think my friend might be anorexic?

so about a month ago my 12 year old friend completely stopped eating she doesn't buy lunch and all she eats for dinner is a sandwich and no breakfast either every time we ask her if she ate she gets very defensive and always says yes but i know she didn't she actually told me shes doing this because she wants to loss weight shes only 12 and very tall shes the perfect weight but she just refuses to eat HELP!!!! i don't want her to get sick :'(