You can verify the diver by following the steps on NAUI's website. I doubt your dive partner dupped you.
NAUI's online "verify a diver" is very incomplete as far as the database goes, especially with regard to Canadian certifications. At least though anyone can check it, without having to sign in like the PADI system.
Just as an aside, we were recently told that some places like Egypt (I think it was Egypt) will not even accept the NAUI cards without a cert number on them cuz they can't verify that the card is legit.
As was mentioned previously by NWGratefulDiver, NAUI just recently started putting a certification number on the c-cards, with each card received of course having a different number. Previous to that, the card only had the instructor number of the issueing instructor for that student. Just use that number if you need to I suppose. I doubt anyone is actually checking the validity of these things anyway.
Bill