I don't understand guys

I have this guy friend, his name is Steven and I've known hin since 3rd grade. We like eachother but never go out, we just mess around with eachother. Everytime we hang out it almost always leads to sex. Well yesterday this girl who's friends with him asked him out. But he texts me and wants to "play". I told him it's not a good idea because this girl really likes him and I would feel bad. He tells me not to think about it because he doesn't really care about her.....WHY WOULD YOU DATE SOMEONE YOU DON'T CARE ABOUT?!?!? So I don't know wheather to tell his girlfriend or not. I don't want to ruin my friendship with Steven but I think his girlfriend should know that he's willing to cheat on her. Help?