Heartbroken Ike

So the guy who I hooked up had a girlfriend that I had no idea about. I even asked him and he was like no there wasn’t anyone. And he lied to me and said that he had feelings for me. And I’m just heartbroken because I really thought he was a good guy. But I also feel dirty and guilty. Should I tell his girlfriend? Not to break them up. But to just let her know. I don’t know what to do.