He has no control over what the ex-wife says or feels, and nothing he says or does can change it.
My wife really struggled with a very similar situation for the first several years of our marriage. My ex-wife is diagnosed Narcissistic Personality Disorder, and my current wife has a flash temper who is always quick to engage in pretty much anything. She social media stalked my ex, always asking me questions about her and our relationship, comparing herself to her, even went so far as to cut her out of old photographs at one point. That combination of my wife and ex wife made my life very difficult.
The thing is, I had nearly 20 years of building the skills to deal with my ex-wife. My current wife did not.
Every communication my ex and I had was through text or email because my ex would never keep to the topic of discussion, always tried to derail and deflect. I wanted to have a record of everything she said for when we would inevitably end up back in court. And we did, several times. I also made sure to use as few words as possible with her. Only responded to questions about the kids. Ignored everything else. As many yes and no answers as I could. No flowery language, or filler words or anything. As minimal engagement as required.
It was a lose-lose with my wife...not telling her about communication with my ex, and then talking about plans with the kids always ended with...So you did talk to her? Let me see the conversation. Tell her about talking with my ex-wife, let me see the conversation.
My ex always had passive aggressive, or full on aggressive, or snarky, nasty things to say about my current wife, and I refused to engage my ex with that. Only engaged about the children. Sure, I could have gotten mad at my ex, told her to stop, yell at her, but to what end? It would have been nothing but engagement for the sake of engagement. It would not change my ex'es behaviour, and it would just feed her need to sew discord.
The problem with me not engaging was my current wife saw that as me not standing up for her, and picking my ex over her. I did engage my ex a few times over things she said about my wife, mainly to humour my wife, and when it inevitably did not change my ex'es behaviour, back to square one of dealing with a vicious ex wife, and wife angry that I could not control my ex.
After a while, I stopped validating my wifes feelings of anger towards my ex wife because my wife was the one who unnecessarily and purposely set out to stir those feelings up. I divorced my ex wife to get away from the discord, and I was not going to be remarried to that by proxy.
It was a rough first few years with my wife, and dealing with my ex, and it almost broke us up a few times. At one point, my wife started directly dealing with my ex. That was hell, and in the end, nothing had changed. It was just a load of screaming and yelling and nastiness that changed literally nothing. Just a bunch of time and emotional energy spent on anger for the sake of anger.
Not sure there was ever an aha moment with my wife where things suddenly changed, but gradually, my wife came to see the situation with my ex for what it was rather than what she wanted it to be. I think it slowly sunk in that she couldn't change or control my ex and what she said or felt. That my ex was was never going to have some moment of enlightenment and see the error of her ways, and suddenly change.