What would you do if you were in my position?

I have a guy friend, and he told me he cheated on his girlfriend multiple times. I don't know who the girl is, and I don't even know her name. He moved from here, and after that he started dating with a girl I don't know. What should I do? Should I try to teach here in some way or shoud I mind my own business?
What would you do if you were in my position?
Post Opinion