All these answers are good for a girl, but how do you say it to a boy? My boyfriend is 24 and (much as i hate to say it like this) really really fat. I've tried the nice method, i even did the harsh method (and hugely regretted it afterwards) but he is doing a sport course and keeps talking about when he's lost weight, but the fact is he doesn't lose weight. He started piling on the weight because he just never cut down his calories after he stopped training cos of sports injuries, and he does go to the gym but because of his injuries playing up he doesn't go as much as he needs to, he'll go something like every day or every other day for a few weeks and then it'll be his ankle or his shoulder. He has a go about my weight and yet he is more than twice my weight and i've been losing it. I snapped the other day when he had ago about my poor circulation and said 'I may have poor circulation, but at least i'm not bordering on morbidly obese' It really upset him, but its true, i just don't know how to get through to him, i know he has trouble with his ankle and his shoulder but they are not gonna get any better if he doesn't lose weight.
But to the OP, tell you what worked for me. I was in yet another mood with my boyfriend and the girls decided to take me out and cheer me up and instead of the usual go out clubbing and for a drink, they took me to a dance class. It was really fun and now we go every week, it helps to keep off the weight as long as you eat sensibly and its a lot more fun than the gym. Next time you see her going to McDonalds stop her and say 'why, don't we go here for a change' and choose somewhere like Subway or somewhere that does healthier stuff.