I think the point is she shouldn't have to try EVERYTHING. If she's healthy, meaning her medical tests come back fine, and she's getting exercise the amount of work she might have to do to LOOK thin might not be worth it. In fact it might even be counter productive and make her less healthy.

Of course it's far easier to complain that she's unfit and that everyone loses and gains weight at the same rate than to say you don't like the aesthetics of her body and she should lose weight because being fat makes her unattractive.