Hindsight tells me I was very nearly anorexic as a very young child - what I put in my mouth was the ONLY thing I felt I could control - my home was chaotic, I was an only child, there was alcohol and emotional abuse and an extremely controlling mother. I don't know what saved me but I am grateful. Of course now that I'm over 50 there is no danger - absolutely no danger at all that I will ever again be too thin!
At least in my case it was a control issue. You are ill-prepared to address the root problem for your friend but you can offer support and that may be what she most needs. Just offer her that - it may be her first step.
Hugs to you for your heartfelt outreach!