A yo bro, I've been in this situation before. Not be negative or anything but You're going to end up confused and stressed out over this. And since you're a nice kid people will advantage of you. My advice is, if you can, try to take things slow and don't get too attached to her just keep it friendly and if you feel comfortable then keep it going with her. You already mentioned your friend isn't that great. He fucked her when you still liked her so you shouldn't care about his feelings since they broke up. But main thing to make sure is that she doesnt just use you or goes back to him after a while that is what you need to watch out for 

There is another option too: fuck her, give her few bitch slaps and piss on her, then say that you break up with her - no regrets and you will get your pleasure.