If you know someone has cheated and plans on cheating on someone how do you tell them

Randomly met someone kind of got involved. Didn’t sleep with him but I know he’s cheated on his gf. I also know that he plans to again. How do I tell someone I don’t know. Do I stay out of it?

Forgot to say I didn’t know he had a gf at the time!

And again I’m not cheating with him. I just know he told me he meets with girls he meets online. I would never be with someone who is married or someone in a relationship.