Yes, my DSD still sees her DM, Sad9999. But things have changed. It was a long process, which I'll describe.
After she hit her teens and could get to ours using public transport, my DSD told her mum that she could no longer keep her apart from her dad. So she stopped trying. Then for years my DSD had free access to her dad and we never said anything critical about her mum.
During that period my DSD gained a realistic view of her mum's problems. She saw that she was a very unhappy woman and that the problems in my DSD's childhood had been her mum's doing. She became very close to both her dad and me.
But we never ran her DM down, which meant my DSD felt safe to discuss the problems she had with her DM and ask our advice. Her relationship with her DM improved and eventually, by some miracle, her DM started to mellow about the whole situation. She stopped hating us. Saw us as OK.
Then my DH got terminal cancer and died. It took a year. After he died my DSD's mum told her "Prawn and her family won't want you any more."
But that wasn't true. I love my DSD, and she's sister to my DC. My parents love her too, and my DBs. We all do. And of course she was still part of our family. My parents helped pay for her to go to university. So nothing changed.
That was the end of the bitching. My DSD's mum kinda melted. She calls me her DD's second mum now. She calls me her friend.
She can still be a total PITA, not to me but to my DSD. Manipulative, controlling - just like your Ex. So when she's kicking off my DSD rings me and gets it all off her chest, knowing that I'll sympathize but will never breathe a word to her DM. And most of the time she's fine.
My DSD is married with DC now. I did the Father of the Bride speech at her wedding and her DM complimented me on it.
It's been quite an experience. What they call a journey these days. And worth all the effort and heartache. I love my DSD.