A. Hire him a hooker to help him forget all about the other girl

B. Sneak around with her behind his back and keep juggling it until it all falls apart and he gets super pissed off

C. Screw your friend and do whatever the hell you want

D. Give up the happiness you would have had with her for your friend's, after all "Bros before hos"

As you can see, they're all very good options. My favorite is A.