My partner of over 20 years did something similar fairly recently. Met her at the gym. He used the same script around having a friend and how much I would get on with her (hate that so much) He still thinks I would for some reason, I honestly can’t think why. I find it quite offensive that he thinks we’d have anything in common, I find her arrogant and awful as a person 😂 Then when I found the reams of messages, inside jokes, her playing damsel in distress and him playing white knight - I was understandably upset but he kept saying he ‘couldn’t believe this was happening over a friendship’. The only thing that got through to him was referring to our friendship of nearly 25 years. Our relationship, the life we’ve built together, our child, our new baby - none of that got through, but referring to the loss of our long friendship made him see a bit more clearly that it wasn’t on.
He still thinks he did nothing wrong but has at least stopped messaging her. I don’t know if they see each other in passing in real life, but for now I’m content that there’s no trace of her in my home. I’m heavily pregnant so I’m picking my battles carefully (that’s a bit of a pattern with them too - doing it while we’re pregnant).