He never really loved her like that in the first place. She could work out to remedy that situation if she's opposed to surgery. They could come to a compromise. But the fact that she's not overweight or obese and it's an aesthetic issue with the appearance of her stomach, only 12 wks out.. there were underlying issues with their relationship and that was a reason enough to make his break from her. If it wasn't her stomach, it would have been something else wrong with her.
The fact that she just gave birth, and it's not like she's one of those women who are lazy or satisfied with being out of shape- shows he just wants an out and needed to feel justified in doing so. I would have got the surgery and dumped him off the strength that he's being so inconsiderate about it. Avoid any future problems bc he already checked out and his words show he doesn't care , neither would I .