I swear to Anoia that I will murder anybody who says 'Bros before Hoes', or has already done so.
Seriously. That's the stupidest rule I have ever heard and it does not exist. never has done, and never will do. Anyone who subscribes to that is an idiot.
The whole thing is up to you. If she's interested in you, and you like her, then go for it, regardless of anyone else. You aren't controlled by anyone, she isn't controlled by anyone, and your friend has absolutely no right to decide whether or not you can date someone just because he also likes the same girl. That would just be wrong.
So I say, do whatever you want, and anyone who tells you otherwise isn't worth listening to.