She is married to him and has her own experience of living with him.
If she is happy and he is good to her, she isn't going to change her opinion of him just because your experience with him is different. You will just come across as a bitter, jealous, batshit ex-wife.
If she is unhappy with him, she will already know what he is like, so you don't need to tell her.
I was once very happy with my ex-husband. He was kind, loving and gentle. There was nothing he wouldn't do for me. We adored each other.
Then he feel out of love with me and - to me - became a different person. I didn't recognise him anymore. He was still a "good" person, but the generous, loving man had gone.
Of course, he hadn't changed really. He was the same man, he just didn't love me anymore. His new wife now has the man I once had. They love each other. Why should I want to spoil that, unless I feel bitter and jealous? Even when the pain of him leaving me for her was raw, I would not have said anything to her about it.
Maintaining my dignity was important. Twenty years later, I'm glad I let him go without drama. I feel nothing for him now and - as far as I am aware - they are still happy together.
Accept that he doesn't love you anymore and that affects your relationship. Also consider how your behaviour affects the dynamic in your relationship. The very fact that you feel the need to badmouth him to his wife suggests toxicity on both sides of your relationship with your ex-husband.
Leave their relationship alone and concentrate on being happy with what you have.