Try suggesting things initially that she may find as fun. She may not like the gym. But she may like to take walks, go for a jog, riding a bike in a scenic area, go rollerblading, hiking. Anything to get her moving. And these are all things that you guys can do TOGETHER. That is where you will win the points as a good boyfriend. Your spending time together doing something. Initially most people if they are sensitive about their weight will not want to set foot in the gym where all of the "fit" people hang out . But if you guys can start doing those things a few times a week, she will start to notice the difference in herself. She will loose some weight, her energy levels will go up, and so will her self confidence. As time passes she will like what she sees, and then maybe want to go to the gym, and eat right. But it will take baby steps first. Hope it helps at all.