What to do when your friend likes the same girl?

I had the same thing with my friend and I simply told my buddy jokingly to leave her for me and he did. He even helped me to get friends with her. Just talk to your friend and ask him if it's okay with him. If he didn't take it very good, I suggest you simply back off a little or sort things out with your buddy first because buddies are buddies and girls are not worth losing a good friendship.