I don't know.
I was in a similiarish position. In my case, the man is my 'friend'.
I know he lives with his partner but one night he came to the pub with a woman I'd never seen before. Got talking to her and it was clear that she thought he'd split up with his partner and now considered herself his new girlfriend.
I asked her if she was sure they'd split up because, as far as I was knew they were still together.
I knew they were because he'd only just been allowed to move back home after spending a week sleeping in his car after the last time he'd cheated on her.
His partner knows he cheats repeatedly so there'd be nothing to gain from telling her but I didn't want an innocent woman getting drawn into the mess.
She wasn't happy and dramatically 'dumped' him in the pub and thanked me for telling her.
He was bemoaning the fact someone had ratted him out - I just said, "Gosh that's awful, yes, who would do such a thing?"
I'm not going to stand by and let some bloke make a fool out of two women. And yes, I did get a sense of satisfaction at him having his arse handed to him in a pub full of people.