Hey Sophie,
I am so sorry to hear that your friend is not doing so well. I would let her know that you care about her and that you support her. Make sure she knows how much she means to you and that you are worried about her. Remind her that you are there for her if she ever needs to talk to anyone.
Make sure you don't talk about food or weight around her and don't act like the food police and try and get her to eat. I also wouldn't comment on her appearance wether it is "you are looking to skinny" or "you are looking so much better" just because this can be very intimidating and actually set her back.
If at all possible I would visit her in the hospital. It is very easy for people with eating disorders to feel alone. Just being there can do wonders. I would also let her know you are proud of her for getting help. I hope your friend gets better soon. No matter what happens never give up on her.
If you ever need someone to talk to feel free to
PM me. I am never to busy to talk and I will do my best to help you any way I can. Take care of yourself.
Lots of love <3 Mimi