My take on this is, get your pal's opinion on the girl first if he's a good pal and he's worth more to you than the lay is. And yeah, what Franco mentions - just do cold approach, and you won't even run into this.
I've had friends hook up with chicks I've slept with, and I've had friends date ex-girlfriends of mine. It's a little bit weird. Some pal tells me he just hooked up with that chick I hooked up with before... and I'm like, "Umm... that's great!" My only main concern is that if she was an ex-girlfriend I cared about, my friend is required to be good to her and treat her well. My ex-girlfriends are kinda like my little sisters, almost... if you want to date her, and you're happy, and she's happy, great; but if you break her heart I'll mess you up, dude.
Obviously, this is different for every guy - both how you feel about it, and how your pal reacts. Me, I won't touch a friend's table scraps (erm, I mean exes). And I'll give a good ribbing to pals who sleep with or date girls I have, like, "Don't worry bro, next chick I break up with is coming right your way for the rebound."
It's an ambiguous topic... there's no set "bro code," but don't piss your bro off if he's a person of importance to you. (if he isn't, well, I guess it's every man for himself in that case)
Chase