I slept with my best friend's girl
Why would you sleep with her?
The girl was a very nice person and a good fuck. It wasn't a good idea from the start
It's awful that you'd sleep with your best friend's girlfriend. He's gonna be really upset over this.
Maybe I should let them break up naturally and take his place after that.
I don't think you should be involved with either of them in any way at this point. Before you cause your friend any more pain.