I'm going to have sex with someone my friend is sleeping with
Why would you do that?
This will make sure she doesn't win.
You shouldn't have sex with someone your friend is sleeping with. You should go find your own girl.
It's not like I'm trying to steal her. I just want to ruin the relationship.
But you are putting that other person in the cross hairs and using them, it's not fair to them. If your friend upset you, tell them, don't do this.