Would you tell a girl that her boyfriend is cheating?

I know I would want someone to tell me. There's a guy I used to hookup with years ago, and one time after we hooked up, he told me he had a girlfriend. All three of us used to work together, so I know her. She's a great girl. He continuously tries to message me, asking to meet up lile we used to and to send him nudes. And honestly I'm disgusted and fed up by it because I'm married now and he still fucking has the audacity to message me. I just think it's wrong to hold that kind of information from his girlfriend. Like I said, I would want someone to tell me. Wouldn't you?