What do you think if I told you I like a guy at my school who is dating my best friends friend?
It feels like it's going to be complicated. Why would you feel that way about him?
Because he's pretty cool.
I think he sounds like a nice guy.  However, you can't be pursuing someone who's dating a friend of a friend.  They're already in a  relationship, and it could harm your friendship even if you managed to get the guy to go out with you instead.
But he can dump her and then we can start dating.  Did you not hear what I said? He's pretty cool.
Oh, I totally hear you, but you should at least wait till the relationship is over and then discuss it with your friend. It would be disrespectful to sabotage your friend's relationship, and it might cause you to lose your friend in the process. You are beautiful, and you should try to find a boyfriend outside of your social circle.