Most Helpful Girl
If he's your best friend, I think he should understand. If you tell him in a cool way, not like anyway to embarrass him or anything; that you've liked her a really long time now before you even knew he liked her. Then ask him what he thinks you guys should do. If you can't go to him outright like that, try asking him just like you posted on gag. tell him you have a friend whose best friend likes the same girl he likes and your friend doesn't know what to do. Chances are, he'll figure out that you're the 'friend', if not hear what he says your friend should do, then tell him afterward that the friend you spoke of was actually you. Good luck!
Most Helpful Guy
The girl is probably only a passing interest but the friendship can be forever (my best friend has known me since I was 16 years old. We still see each other on a regular basis.) Both of you should agree to back off and leave this fish in the sea.