I agree with Lil, be very careful as to what you compliment her on. Saying things such as, you look so skinny in that, you are so tiny, things along the lines of how her body size differs in clothing, try to stay away from that.

Observe her, watch her subconcious actions. Because Just like lil bit, I took those compliments as fuel for the fire. It made me want to lose even more weight. Just be cautious of the things you say and the things you dont say. That might sound weird but, a simple I love you before going to bed or random hugs, things you might not do. You should think about starting

GOOD LUCK!!!