try aversion therapy; every time she says she's fat, smack her with one of these
/jk
seriously I think this obsession with self image is sickening, she should realize that thinking you're fat is NO excuse to avoid social interaction.
REAL fat people get along just fine. She should be happy with how she looks, and if she's not, she should do something about it rather than hide from other people who don't really care what she looks like (seriously, I'm a student and I don't give a shit how anyone looks, let alone myself... as long as we're all clothed and clean we've met the major requirements of being in public) who is she trying to impress anyways?
is this a constant thing or is she menstruating? Sometimes that can... skew one's perspective. If she actually believes this and is consistent about it, maybe she should seek therapy.