Offer to take pics of their next wedding for free?

If he can't find the card, then here's nothing he can do 'cept tell the truth. Hopefully they can get pics from their guests. Maybe he can offer to fix those pics and make them look more professional?

If he really loves being a photographer then I don't think he should change career just because of this one incident - accidents happen; it's not like he purposely lost the card. Good luck to him.