Most Helpful Girl
Maybe explain to him.
Most Helpful Guy
It's compassionate of you to consider your friends feelings.
I would talk to him first, I suppose. Then weigh how much you like this girl vs. how much pain it might cause your friend. Ultimately, unless you were just out for a fling with her and your friend would be devastated, I would probably ask her out.
By the way, it happened to me twice in my life that friends of mine did start dating girls I liked. It hurt like hell. Normally, if I'm rejected I just distance myself. However, I couldn't do that without distancing myself from my friends. I decided it would be awful to lose a friend, so I just had to tough it out. I am still friends with both of them