Let's give the benefit of the doubt. The adult and mature way to do this is that it has to be her decision to eat healthy and lose weight. The OP (or anyone else) can help encourage their partner to lose weight by exercising and eating healthy with them. The can also give them the tools in order to lose weight and build lean muscle. The secret in losing weight is controlling what goes into a person's mouth. If, after a certain period of time, you must make a decision. Either accept it or leave. The leaving part is hard, but if you do not, and she (or he) continues to eat and lead an unhealthy life; you will undoubtedly pay for it in the end. Should you leave, you can at least know, while she did lose you, she will work on herself to eat and live a better life. Chances are, she will love you more than you love her, so it will hurt her greatly if and when you leave. This is unfortunate, as you may love or like everything else about her, but weight is a big issue with men; and I agree. Rest assured she will work on herself to live a healthier life as she doesn't want to feel that rejection hurt again. Who knows, she may come back losing all that weight, but it's a double edged sword, as the resentment inside her will build up knowing that she has to keep a certain lifestyle in order to keep you happy.
I've always said, diets DO NOT WORK. However, lifestyle changes do. They need to change for themselves, not you. You can be patient, six months, a year, whatever you feel is appropriate. But, as a general rule for myself, if I do leave, I tend to try to leave them in a better place than where they were prior to meeting me.
