This is one of those things that genuinely sucks about being female. No matter how much a girl works on armoring herself against it, society as a whole expects attractive women to look a certain way, parades women who do look that way around as models, and disrespects women who don't as, at best, plain. There are subcultures where appearance isn't that important, but it's hard for most people to live their life in those subcultures.
Guys don't have it that rough. I've been overweight most of my adult life, and I haven't had to put up with 1% of the crap a woman in my weight class would. Even in the military, where fitness and proper weight are required, nobody outside the chain of command dogged me about my weight. (Well, there was that one guy, but he got a full beer can upside the head for his mouth.) For the most part, I think that's an artifact of my height (I'm a little over 6') which tends to make people worry that I might fall on them and crush them. Or something; whatever the reason, not too many people in my circle of family and friends have criticized my weight for other than medical reasons.
I wish I had constructive advice to offer chibitoaster other than "get away from those shitheads, they're making you crazy," which is the only thing I can think of that would do her any good. Being a single mom and a full-time student is enough to stress anyone out and suck up all available free time as it is without trying some diet that by itself won't turn her into Sophia Loren...who would probably be considered too pudgy by the assclown aspiring artists these days anyway. All I can really do is be there and say things like this, and reassure her that no, she's not fat, and she looks just fine. I like her personality and appearance just fine the way they are, thank you very much, and wish illness and schedule conflicts and stuff allowed us to get together more often.