I found diet and exercise go together. you have to have something to diet for, besides "only" the slim body. something that gives you more satisfaction. You run, that is a part of losing weight, you also diet, you lose weight, running gets easier, you run more.... (ideally)
also: for "run" you can insert any sport!
But: if she finds she does not LIKE sports and just finds them a nuisance (there's lots of chicks like that), there is literally no hope other than appealing to her self-respect and sence of aesthetics.