I am aghast at the idiocy of this prick.
Why the F does he INSIST on involving this woman in conversations with you?
Solo, have you actually said 'Look, I do NOT want to be involved in the ins and outs of your relationship with your mistress. I could genuinely care less where you leave her, or if you ever bothered to pick her up again after. Do NOT bring her to my attention again, do NOT involve me in conversations about her again, or you will sour the tenuously cordial relationship that currently exists between us.' Tell him that every time you have anything to do with him, he has to bring her into the conversation, either to moan about her, try to get the situation to be under HER conditions, or with HER approval.
Tell him that he needs to understand one thing. That YOU DON'T CARE about her opinion, it is not relevant to any part of your life, nor that of the DC.
She is not YOUR problem, she is HIS problem and as such he is ON HIS OWN as far as she is confirmed.
If you feel the need, tell him it's a deal breaker and if he insists in rubbing your face in it, then you will go contact centre for all access.
Huge threat, but ought to put his stupid thoughts into some kind of focus.
Then, If you happen to be on the phone and he starts to talk about her, about how she is unhappy about this or trying to get him to do that, as soon as the conversation heads off in that direction, without hesitation, terminate the call.
If you are face to face, change the subject, a hefty and uncomfortable, "ANYWAY, not of any interest to me, Change the subject please."
Broken record.