My friend told me today that she wasn't eating?

we went to the mall and i commented on how skinny she was getting (were 13) and i asked her how she lost so much weight so quickly and she told me not to tell anyone but she wasn't eating. she said that she takes the food her mom gives her for breakfast she just throws away and she throws away her lunch at school and she says she only eats like two bites of her dinner because she has to because her family always eats dinner together. i dont know what to do, its obviously not healthy but she wont listen to me. i tried everything i can think of but she won't budge. she says that she's never felt better and that she would hate me if i ruined it for her. i want to tell her mom but she said that when she lost a little more weight she would stop, she said that a while ago and shes STILL not eating and now she is throwing up the food she does eat! im so confused and i dont know what to do so i am pathetically turning to strangers on the internet because i cant even tell my own mom because my friend said that she would hate me forever. please help!!!