Agree. And I think it is a scary prospect for OP to be honest - which is understandable. In a situation like this I do think it takes time for it to dawn that the man you married is no longer the man you thought he was - if he ever was.
OP’s been living with this for a while and has actually been doing something akin to the ‘pick me’ dance, hoping things would change, despite the fact that he’s told her in no uncertain terms that it won’t, that no matter how much it hurts her, he has no intention of giving up the ‘friendship’.
So OP is left in limbo watching the relationship progress and this woman get ever closer to her husband. She has no idea whether this has tipped over into a physical affair, but I think deep down she has to accept that even if it hasn’t so far, at some point it’s inevitable. And to be honest I don’t think it matters. I think his appalling lack of care or even interest in the distress he’s causing OP is more than enough for her to call a halt to it.
Posters are calling for OP to issue an ultimatum. Either he puts an end to the friendship or their marriage is over. I think it’s gone well beyond that point now. He’s allowed himself to get close to another woman to the point where it’s damaged their marriage, a clear indication that OP is no longer his priority in life. He doesn’t have her back the way a life partner should, and the trust has gone.
OP has said he’s a good man and a good father. She’s trying to convince herself that this is the case, but deep down inside she must know that a good man and a good father doesn’t behave like this. He’s been slowing blowing up his family and he doesn’t care. As harsh as it sounds I think OP has to take off the rose tinted specs, look at the reality of her situation and accept that the damage is permanent, and find the courage to end it now