To the people saying 'she must have an eating disorder to be that thin, it's not natural'- being unhealthily thin doesn't mean you must have an eating disorder, there are lots of reasons to be unhealthily thin! She may have another illness, she may be an exceptionally fussy eater, it's even possible that she just doesn't eat much food and has a small appetite. None of these things mean she's a healthy weight, but I know lots of people who are unhealthily thin just because they don't like a lot of foods, or because they don't like feeling full. One guy I know pretty much only eats garlic bread and popcorn, he says he just couldn't stand to eat anything else. One girl I know stopped ice skating, lost her appetite and lost weight- she wishes she could GAIN a stone.
I don't mean to labor the point, I just don't think we can really know unless she says anything. Even if she likes being that thin, she may not see it as difficult to maintain.