Honestly I think its a valid complaint. I think divorce is too drastic, she was probably just trying to scare him. If she stayed the same size, and this guy gained over 30 pounds of fat, She has a legit gripe. Too many times the shoe is on the other foot and its the woman that gets "secure" and blows up like a whale. You dont want to marry someone who looks a certain way and a few years later is a fat slob. False advertising.
Seeing your point. To gain like that he'd have to sit on his ass and endlessly stuff his face. Witnessing it day after day would grind my gears. Man or woman, if you're gonna pull the ripcord like that, you deserve a telling off.
On the other hand, he should take better care of himself and getting rid of her is a part of that. If he had a serious accident or illness, and who doesn't sooner or later, this is not someone who is going to be very supportive through rough times.