It sounds like your girlfriend has some self esteem issues. Maybe she could see someone to help her boost that up. Losing weight with those issues is not going to help anyways as she'll always feel inferior to everyone else. She needs some help in boosting her self esteem first.
I've been overweight most of my life but I never let that get me down. I'm not severly overweight (similar to your girlfriends weight) but I go to the beach and do everything else everyone does. I feel if someone doesn't like me for what I am on the inside then their loss not mine! Since she is your girlfriend you must find her attractive.
Just try to support her and see if she can get her self esteem up. Then it won't matter where she goes as she'll hold her head high!