I'm going to tell a girl my close friend is hooking up with that she's just going to end up hurt in the end.
Why do you think that she is going to get hurt?
That's just what he's like. I want to end my friendship with that guy.
I think that for this you should just worry about yourself. You should just let her figure out things on her own.
Nah, I'm going to make sure she knows that this guy is just going to hurt her again because I don't want to lose my friend.
It's not clear if you really want to warn her or if you just want to have your friend back. Either way, it's not really any of your business.