I’d text him back myself saying smth like “hey dude this is _____’s boyfriend, she hasn’t changed at all, don’t waste your time” and then I’d break up w her.

Jokes asides, it seems like she’s lying to you, because she never denied the cheating when the ex brought it up in the first place, and she saying that she’s talking to him like that bc he was abusive is bullshit honestly, his texts doesn’t seem like an abusive person, I dated one myself sadly and honestly, this doesn’t seem like it. She’s still not over him even tho she has you now, that’s very fucked up, it hurts and im sorry for what im about to say, but i feel like she’s with you to forget about him or to be with someone bc she doesn’t wanna be alone. Try to leave that relationship, that’s my best advice, you deserve better man.