As Kelcyjane3 pointed out, sometimes it just takes persistence and patience....extra emphasis on patience....but that goes with the territory when it comes to relationships.

I hate to be the one to point this out but there is a chance she gained weight due to more emotional or phycological reasons rather than comfort. 50 lbs is a lot of comfort weight. As a woman I suggest finding out what she likes, everyone is different. Yoga, Zumba, Jogging, Lifting, Swimming...maybe she just hasn't found her fitness passion yet. And remember women and men are not so different; Positive reinforcement is always the best route.