This is a fine line to cross. I've been in a relationship where she put on about 40lbs. And it was very noticeable. Of course we had the hypothetical conversation about this very scenario and when that scenario was brought up and she asked me if I would tell her that she is putting on weight that I should tell her. So, fast forward a few years and boom scenario happens and what do I do? I punk out and don't say anything because I don't want to hurt her. Instead I get her ass into the gym every day. I don't think it's right to leave someone in that scenario unless you have made some sort of effort either communicating or physically taking the person to the gym and if that doesn't work out then I think leaving the person is valid. I believe in a relationship with both parties involved that they should try to keep themselves in good health and condition as an effort to better themselves which in turn betters the relationship.