I'm going to fuck with my best friend who just broke up with his girlfriend.   I have a complicated relationship with her.
Why would you do that to your best friend? What's your relationship like with her?
I love him, but I'd also love to fuck his girl
No matter attractive you find her, you shouldn't sleep with your friend's ex-girlfriend.  This will make things very awkward for all three of you, and it's not worth risking a friendship over.
I don't give a fuck. I'm going to fuck with her.
If you really consider him your best friend you will put an end to this and respect what he went through.