Because of my morals, I'm going to tell a friend the girl he's talking to cheated on her last boyfriend.
Other than your morals, why are you going to tell that to your friend?
I think he deserves to know because maybe she is cheating on him too
Yeah, he probably does deserve to know, it's okay to want your friend to avoid a bad situation. If you don't tell him he will probably get cheated on too.
Yeah, that's probably true.
Telling your friend that they are with a cheater is the right thing to do, so I support your decision.  Showing high morals is a great character trait, and your friend will appreciate you for telling them.