It's always important to let her know that you are there for her. Try not to comment on her weight, EVER. It only makes anything worse, even if you mean well. The best thing you can do is probably do lots of research on eating disorders. That's what I tell my family, and it means a lot to me when they do pick up books on eating disorders so that they can better understand what I'm going through. Life Without Ed is a good place to start. It's a very easy read and gives some good insight into the ED world.
I think it's awesome that you're already trying to learn more. Keep it up, and don't get discouraged if your friend seems to be pushing you away.