Anyone who says 'Bros before Hoes' is an idiot and I will hunt those people down to the ends of the world and beyond and shove various types of grenade down their throats before stepping back and allowing my pet wolves tear them limb from limb, just as the grenades explode. Oh, and I'll make sure acid is dripping on them during he ordeal as well, for good measure.
Avykins, this applies to you too.
Now that's out of the way, I'll be serious. Take my life as an example. I used to fancy one of my housemates, but didn't do a damned thing about it. Now she's dating one of my other housemates. If they were to split up and she decided she liked me, I'd definitely go for it. Obviously since I get on with them both, I'd try and make sure it was okay with my male friend first, but even if it wasn't I wouldn't let that stop me. When you split up with someone who are you to say who they can and can't date? You don't have that right and if your ex-partner wants to date your friend then you can't stop them. You may not be happy, but it's not your decision to make for them.
Hence, OP, I say go for it. If you like this girl, go on and ask her out or whatever. Maybe let your friend know, just because it's still good to be honest with him, but don't let him tell you what to do, and don't let him keep a claim over her like she's a piece of meat. At the end of the day though, it's up to her and you, not your friend.