There are more problems here than just the house.
I think it's great that the divorce was amicable, but the keeping the house in both names was idiotic. It seems to me that John still feels like his primary relationship is with Jane, not with Mary. The kitchen remodel just screams that he does not view Mary as an equal partner with him in his primary relationship.
It is true that Mary should have insisted that the house situation get settled before she married John, but that ship has sailed.
If they are interested in salvaging the relationship, I think the best thing is to start fresh by selling the house and John and Mary buying a new one. But I think they need counseling as well.
I think it's great that the divorce was amicable, but the keeping the house in both names was idiotic. It seems to me that John still feels like his primary relationship is with Jane, not with Mary. The kitchen remodel just screams that he does not view Mary as an equal partner with him in his primary relationship.
It is true that Mary should have insisted that the house situation get settled before she married John, but that ship has sailed.
If they are interested in salvaging the relationship, I think the best thing is to start fresh by selling the house and John and Mary buying a new one. But I think they need counseling as well.
If John is so protective of his former wife, then he should have stayed married to her. Is Jane the one that initiated the divorce? Not that it matters, but I'm curious. 
He and Jane had this figured out and amicably agreed to LONG before Mary entered the picture. Why should he go back to paying on a mortgage when he's already done paying and he only has to pay 1/2 the rent on his ex-wife's apartment?