Two banks still could have verified everything was ok. His bank could call the issuer of the cashier's check to verify its authenticity. For anyone that didn't know, you can't get a refund on a cashier's check without returning it to the bank. You lose it, it's your tough luck -- no excuses or stories to make the check void. Once it's issued, it's valid.
My brother bought a Supra from a guy who was this paranoid. He actually suggested we take a bag of cash to FL to buy the car. Yeah, right! We ended up doing a wire transfer while at the seller's bank so he knew when it was credited.