Friend is in need of help

(australia)

Hi!
my friend thinks she is fat, but me and my other friends try to make her tell her that she is not fat.
Just a few days ago she told every one of us that she is going to try to be anorexic. She is doing this by reducing her meals by 2 a day, exercising excessively and just not eating during recess and lunch.
How can I help my friend. I am worried that it will have a big impact on her life.
From Michelle, 15 year old