What should I do? Should I be her friend? Should I steel the guy away from her? Should I tell her how I feel?

This chick is your -best friend-. Is any guy really worth losing your best friend over? Your friend has done nothing wrong, she hasn't lied to you, she only told you what was true: that he liked her. If you went after him, you'd be betraying her friendship on the off chance that he would take you instead of her.

I've been where you are, and I know it hurts. By all means, talk to your friend, because it's not worth it to lose her over some guy. The more you communicate about this, the better things will go. But do NOT try to steal him from her, or you will probably lose the friend -and- the guy. When I was in this situation with my best friend, we just talked it out and decided that we would let the guy take the lead. Whichever of us he picked (if either), we would not blame the other for it. The guy came and went, but we are still best friends one year later.