You're right, this is a "weird situation." You have to decide what's more important; maintaining two friendships with your guy friend and crush or risk losing both friendships if you decide to pursue a relationship with your crush. Only you can decide.

the best man will prevail

People do not get to mark women like fire hydrants, and claim ownership of them because they happen to think they're attractive. In a perfect world, everybody would understand that, and crushes would not come between friends.

Unfortunately, some people do not work like that. Girls (or boys) regularly drive friends apart, and petty crushes can ruin otherwise lifelong friendships.

Either way it goes, I think your best course of action should be to discuss this with your friend. Hopefully, the two of you agree that your friendship is more important, and that you're both mature enough to remain friends should one of you end up dating her.

Your friend admitting his attraction first does not give him priority, and you being close to the girl does not give you priority. This can be settled in a number of different ways, and I'll leave it to the two of you to decide. Should your friend (or you) demand to be the one to ask her out, that reveals a significant level of selfishness. I would normally tell you that such a friend is not one worth keeping, but I don't know his positive traits, or yours, or your history together.

Both of you need to be reasonable. This is not the only girl in the world, and there's a 99% chance that nothing will ultimately come of whatever relationship one of you may eventually have with her.

I'd suggest talking to him, and then both of you talking to her. Explain the situation, and then give her a day or two to decide which of you she'd like to date, if either. She might have a friend who is available, and the four of you could have a really good time together.