Dig a big hole in your basement and push her in. Feed her a little every day and to avoid wrinkles and saggyness tell it to rub the lotion on it's skin.
Seriously though the issue here isn't about vanity. It is about repect for each other's values. Instead of geting fat it could be about smoking, getting tattoos, doing girl!s nights every week, exclusivity, swearing, picking your nose in public or whatever...
He says he clearly stated at the start of the relationship that he likes skinny girls. She stopped trying to be skinny after they started going out. She made a choice just like starting to smoke again or whatever.
I understand that it can be hard to keep the weight off if you don't work at it but the issue is that she is not working at it. if she clearly stated that she liked clean shaven men with short hair who didn't smoke and the OP grew a beard and dreads and started smoking it would be exactly the same.
Yes "thick" people can be attractive to other people but he isn't one of them.
She is either lazy, was recovering from an illness when the OP met her, suffers from bad genetics and poor willpower, doesn't care about her man's values and is being disrespectful, or is dumb as a box of hammers and doesn't realize her man is finding her less atractive.
It is up to the OP to decide which ones she is and whether it is worth trying to fix it depending on how he feels about her. I would suggest not because lazy, dumb, no willpower or disrespectful are all deal breakers IMHO.
I guess my only question for the OP is why didn't he show some leadership earlier on in the relationship and make his values clearer. "oh honey, I love how slim and sexy you are", "you are perfect the way you are, "you are so beautiful I wanna rip your clothes off rigjt now" etc etc.
You don't always need to be all macho ten commandments to get your point across "thou shalt not gain 10lbs or I will dump thee" although it does avoid confusion.
At the end of the day the OP has to decide whether he can accept this aspect of her because the rest of the package makes it worth it or throw her back in the pond.