ReneBumsWombat I was referring more to the "friend of a friend" with that. But even if this third party knows everyone in question, it's still disrespectful, indiscreet, dishonest and manipulative.
Tell or don't tell. But if you do, be honest and straightforward about it. None of this wangling in other people so you can pretend it wasn't anything to do with you. Nobody wants to know that the OW, even if an unwitting one, was sharing this kind of stuff around.
I absolutely disagree from a moral perspective. This man is endangering peoples' health with his lies and probably their mental health too.
The OP owes him no duty whatsoever to keep his nasty little secrets.
People like him do tend to get an awful reputation sooner or later. Their modus operandi depends on people saying nothing and being complicit in his lies, sooner or later this inevitably breaks down and its better the more people that know the better, so he can be avoided.
If you sleep around and cheat, people will gossip about you sooner or later, particularly if you are so blatant about it on social media as this guy.
He literally tells lies to get women to have sex with him. The OP is very unlikely to be the last.