I'm going to hook up with a good friend's ex girlfriend
What is your reason for wanting to hook up with your friend's ex?
Yes, he made out with another girl and I've been so angry at him.
It would be a nice idea to just ask your friend politely if they is okay with it. Things get more difficult when it's your friend who got dumped, so stay in touch with their feelings, too.
I'm going to fuck her.
This is probably a bad idea. If you want to keep your relationship with your friend strong than you should really talk to them before doing something like this. This action seems pretty insensitive towards them.