I think every girl goes through this at one time or another. It's always a shame when they do think these things though... society has a lot to answer for!! Anyway, I would try and listen to her, ask her why she thinks she is fat and try to give her your reasoning. The worst thing you can do is say she is stupid or disregard her feelings, she obviously thinks this for a reason. It might be someone said something to her and at this age girls can be quite impressionable. On a different note, watch her eating habits, this can develop into something more serious (Anorexia or Boulimia) and you want to catch that early on.

Hope this helped some

Good luck