When women think they are not thin enough, best thing is to tell them they are too thin! Our society shoves an unreal image of women daily. Less than 2% of the world population looks like that naturally and the rest are airbrushed and photo shopped. Tease her and tell her she needs to put on at least 3 or 4 pounds to fatten up. Reverse the way she see's herself and perhaps she will see she is perfect as she is and does not need to live up to an ideal that is unrealistic. That should help her avoid eating disorders too! Seriously...every time she says she is not skinny enough, tell her she is too skinny! She will cheer up and may not analyze her size so often which undermines her over all confidence in who she is on the inside!
I totally know where you're coming from, compliment her randomly, like "you look nice today" "cool shoes" or "that looks really good on you!" also help her pick out outfits with her, go shopping with her, and/or help/teach her how to do her hair in cute buns/ braids :D