i used to have an eating disorder, and i went into 'recovery' too.
i hated it whenever people talked about my appearance to me. you're looking so much better, you look nice, etc just came across as ******** patronising lies. whilst YOU may think it helps- keep it to a minimum. say it once or twice, but focus on how glad you are that she's healthy again.
support her, take her mind off of things. if she's away for long periods of time..make her care packages. include books, letters, drawings, etc. try to avoid photographs of her/you together because it'll just make her upset that she's not THAT size any more.
take her out to do something fun- i ******* hated shopping afterwards and hated the typical coffee etc. because even though it's coffee, in your mind you're still comparing calories etc. i'd take her to the cinema, stay in and make stuff (which sounds really lame but it can be fun!), watch movies, go out and do stuff like ice skating/go carting/whatever (which might be too much if she's still feeling a bit rough but maybe for later?) or visit places. a lot of people will be judging her/being harsh so just honestly be really kind and loving about everything and she'll appreciate it lots.
reassure her that she can trust you with everything and that even when she's down and upset she can talk to you. NEVER make judgments or think well that's silly, say that you can understand why she's thinking that but she needn't worry etc and try your best to help. ask her- or maybe just see- if she just wants a friend to hang out with, or a friend who will know everything and help her along, and is a confident as well as a laugh?
also try to keep her out of massive social situations which will spike her anxiety. i found that going to parties etc just made me want to die, as opposed to taking my mind off of things etc.