I have a friend who I suspect if her useless ex ever got a new partner, tha tpartner would think of her like you think of your DP's ex.
She has refused to facilitate contact with her ex at various times. Why? Because he is unreliable, regularly doesn't turn up, treats her and their DS badly. She has always said he is welcome to see their DS at any time, but due to his behaviour, she does not always step up to arrange it and she refuses to allow him to see their DS in her home anymore. In his head, she is the baddie who is keeping her away from his DS. Similarly, when he has gone AWOL for months at a time, if he returns and she does not move heaven and earth to facilitate contact, he blames her.
He has always been a pretty weak dad. Long before they broke up, she spent her days on edge if he was looking after their DS because she knew the chances were that DS wouldn't get a proper meal, would be ignored in favour of his dad being on the phone or outside smoking. During the breka up fase, she realised how much weed he was smoking so was constantly worried that he'd be driving with DS in the car while high and she wouldn't know.
The result is that yes, sometimes she tries to put things in place that, objectively, are ridiculous. eg she had a meltdown when he bought a new car because she didn't think it was safe enough and because he had to get a new car seat that in her view was not sufficient. But that anxiety, while incredibly frustrating and I'v ehad to gently tell her to dial it back a few times, is completely understandable in light of the history.