He needs to get help from a lawyer. If there are any witnesses to what happened, he can try to get them to back up his story. He and his lawyer can find out where and when the credit card was lost or stolen, as well as if it was used in other locations, and see if they can prove that the defendant was not the only person who possessed used the card. If the card was used elsewhere, they can see if there is any security footage of the person who used the card.

All in all, though, absent some luck, it will look a lot like the person who handed the bar a stolen credit card is the person who stole it, and "A stranger wanted to buy me drinks, then handed me his credit card and left", is not a particularly compelling defense.