I'm going to hook up with a girl my friend told me he had hooked up with.
How many times has your friend ever hooked up with this girl in the past?
He hooked up with her twice in the past.
I know that you like the girl your friend has been with, but trying to go after her when your friend obviously likes her is wrong. Find your own girl.
Hey who is to say that he actually likes her? I mean, he's probably interested in having sex with her again and doesn't really care about her.
Whether or not he cares for her is beside the point. It's really in bad taste to step on someone's toes like that.