Overweight girlfriend problem?

My girlfriend is 30 y/o, 5'4", and weighs around 135lbs. We both know she is overweight & looks it, and needs to lose about 15-20lbs. She's been trying...goes to the gym 3 times/wk, walks every other day, doesn't eat much most days.... But about once a week, she'll just pig out! Eat junk or over-eat at dinner...sometimes eating as much (or more than) me. I think she's sabotaging her weight loss by these once-a-week binges, but she says she's just hungry from nearly starving herself all week. But I told her if she wants to be skinny, she needs to think like the Hollywood- types. Think of food as bad, or the enemy. I've actually had to take her plate away when it looks like she's about to eat more than her allotted calories for the day! It's like she loses self-control, and I have to do it for her! How is she ever going to be hot if she keeps over-eating like that? What can I do? I want her to be thin, but I especially don't want her to get any fatter than she already is. My brother refers to her as "chunk" behind her back & my friends periodically ask me why I continue to live with her & tell me I can do better. I love her, but I know they're right. How else can I help her (make her?) reach her goal?