when we are talking about them letting themselves go... what ARE we talking about here?
neither my SO nor I are the same weight we used to be in our 20ties. heck I gained about 15 pounds over the course of 2 decades - still well within healthy range, but i don't fit into tailored clothes I used to fit into anymore - still fit stretchy stuff like sweaters though, he gained about 20ish? that's with us working out and generally eating healthy with occasional junky treat to ourselves. as you age, metabolism changes, our bones change, so many things change.. even if you look at celebrities whose job is to look a certain way, you will see that even the ones aging super gracefully, are still looking a bit thicker. regular people who don't have working out and personal chefs as part of their jobs? changes in body are all but inevitable.
moreover - they could have undiagnosed illness, that is causing weight gain despite efforts to stave it off.
but if they just stopped caring about themselves all together? then you have to figure out why. both of you. and ultimatum aint it. there has got to be an underlying issue here. figure what THAT is and then decide if its a dealbreaker for relationship. weight gain is a symptom.