Do you know she is saying these things? Or is he just telling you she is? Because he will likely be lying to you both about eachother.
Regarding her, if you meet in future (eg: because she's going to be in your kids life) I'd probably say 'I ask moving forwards that we both attempt to meet eachother with respect and decency, when our paths cross. Instead of allowing a man to play us off against eachother like pokemon cards. Because, life's too short for that. Sound like a fair plan?'.
Also 'I'm not sure that the fact your opinion of me is formed solely on what my ex says, paints me in a fair light. Are all your exs lovely about you?'.
I would act warmly towards her even though it might be tough. Maybe say things like 'gosh, this is such an awkward situation isn't it, don't worry, it's all good'.
Yes it's going to be hard but, in showing her consistantly you are a nice person (to her anyway), you undermine what he says about you.
However, I would avoid spending too much time in either of their presenses if possible. The fact that he's ghosting the kids is actually a good thing. He's a prick. Less time they have to spend with him, the better.
Also, I'm not saying to pander to her if it turns out she is a total bitch (and it's not just that he is manipulating her). Just, keep it classy.
You're not a bad person. Expecting him to parent doesn't make you a bad person either. But id ease off for now. Just make sure to get child support set up.