Presumably your buddy asked you to be his best man because he values your friendship and trusts you to play this special role at his wedding. So you’ve created an opportunity to prove yourself worthy of his trust. Yes, you are required to replace the band, just as you would replace or repair anything lost or damaged while in your care. And you must tell your friend what happened. Maybe you have time to replace the ring, and could find an identical one, and no one would be the wiser — but do you want that nagging at your conscience for the rest of your life? Do you want to be nervous throughout the whole ceremony, worried that someone will notice it’s a different ring? Do you want the groom to discover what happened later and wonder what other secrets you might have kept from him? Your friend’s wedding should be a joyful occasion; don’t spoil it by letting an act of carelessness lead to lingering regret or resentment.
As you think about what to do, take a stroll down memory lane to junior high school English class and read “The Necklace,” by Guy de Maupassant.