Several thoughts come to my mind: Maybe he is mentally deficient. Maybe their relationship was on the rocks and he didn't see her much. Maybe he was in on it.
I keep wondering how this woman could do this alone--I guess she could have, but it sure seems like there might have been some help.
Usually the woman does this alone. She has invented a pregnancy or is hiding that she miscarried. Men just don't have motherhood obsessions like that.