Open up any issue of "Vogue", "Elle", or any fashion magazine to see where young women get this from. This is what popular culture tells them what is attractive - super thin and even addict like looks. There's more to it obviously.
One thing is control - she may feel that she may not have control over alot of things in life but her weight is one thing she can control.
Back in my single/dating days, I was regularly appalled at the fact that the *most attractive* women always thought their hips were too big, etc. To put it another way, the GAY fashion industry, which picks BOY-like women as models and puts them on a pedestal for all to emulate, has convinced most women that their normal, sexy, feminine body curves are unattractive! Often I could not convince them otherwise.
It may be a satanic plot to set the standard of physical fitness so far out of reach that (a) women who care enough about fitness to work hard at it, will destroy themselves shooting for an unnatural standard, and (b) many other women will try and fail, try and fail, try and fail, and finally give up on physical fitness completely, thus contributing to our worsening national obesity epidemic. Either way, the anorexic/gay standard is the enemy of real fitness and real beauty.
I am certainly not a professional so defer to whatever the professionals say, but it seems to me that if you could just expose the fashion-fag lie for what it is, and persuade her that her natural body curves (assuming she wasn't overweight before) are more attractive to heterosexual men, that might help. Maybe even ask her, "Do you get asked out *more*, or *less*, since you lost all this weight?"
"One thing is control - she may feel that she may not have control over alot of things in life but her weight is one thing she can control."
It may also be her way of rebelling against something. I wager that this rebellion did not come out of thin air and that its agents came from within her college.