I'm going to hook up with a girl that I know my friend is interested in
What makes you think that your friend is into them?
He told me about it and he was acting so erratically. He kept texting her and even invited her to different places. I just want to show him who's the boss.
It's pretty rude and inconsiderate to want to get involved with a person that your friend is romantically interested in pursuing. If you end up hooking up with that person, your friend's emotions will be hurt and your friendship will suffer. Rather than go behind your friend's back and hurt him, it would be better to find someone else to date that your friend does not have an interest in.
W/e. She's mine now.
I think that this is going to end badly. I think that you should consider your friend, it is not kind to sleep with some one that your friend has expressed interest in.