was this "mate" fairly well built despite being lean?
Reason I ask is I'm 5 ft 7, slender built but with a bit of flab from being unfit and I weigh 9st 5. 11st 7 for being 5ft 9 is nearing overweight if you look at BMI. But if he was a tad toned, that would go towards accounting for it.
By point is if he's gone from a fairly toned lean guy, to an overweight guy, gaining 5" across the middle, it's clearly quite a physical change. It's not her fault that her physical attraction to him has lessened a bit as frankly, he's not got the figure she was attracted to in the first place.
Clearly it's not his fault as he's been in an accident, but now he's recovered it would be good for his health to get into shape, does he not miss his old body at all? I'd have thought so...
If he's totally happy the way he is and isn't going to change, then there's a real problem. In all likelihood her resentment of his new figure isn't going to go away. She'll end up dissatisfied so really if he doesn't want to put the work in, he may be better off finding someone who is fine with his physical appearance as it is now.