So this is your partners ex-wife you are talking about? Who has told you he emotionally abused her and made her life a misery? Does this not worry you?
If this is the case then I suggest you read up on emotional abuse pretty quickly. How it creeps up on you insidiously, the boiled frog analogy, living in Fog (fear, obligation, guilt) - particularly when you have children and are trapped through lack of finances. Your self-esteem is rock bottom and leaving is pretty terrifying when it means leaving your home and breaking up the family. Everybody else thinks he’s a lovely guy....
I have been there. It took me several years to get out. I am bitter about how he treated me and how he treats his new partner so much better. Hopefully he has learnt from me leaving but I don’t think a leopard changes his spots.
We were together many years, like any abuse you don’t get over it easily. But I am genuinely, 100% much happier now than I was for far too long with him. I am free to do what I want, live where I want, buy what I want, get a pet. Etc etc etc. I am fabulously happy, honestly.
If you are referring to your partner’s ex-wife/future husband please be very, very careful. I was happy for the first few years else I wouldn’t have married him. Then when he had me he changed. It’s not uncommon sadly.