How to help my best feiend!?

Kerstin • 18 y/o - taken
Hey i'm 18 and i know my best friend (Girl) for more than 10 years..
​She suffers anorexia (she Dosen't look like an anorexic... She does a looooot of Sport and hardly ever eats.... I only know that she likes eating whem she's with me...)
​The Problem is that i don't know how to help her... We don't go to the same universitY so we're not able to spend all our Time together....
​How can i help her ? I know her anorexia is severe,although noone would see it if you look at her!?!?