Sorry, but if your customer wrote his PIN on the card - it was then stolen and used - the bank will still be required to reimburse.
This is from the commentary:
2. Consumer negligence. Negligence by the consumer cannot be used as the basis for imposing greater liability than is permissible under Regulation E. Thus, consumer behavior that may constitute negligence under state law, such as writing the PIN on a debit card or on a piece of paper kept with the card, does not affect the consumer's liability for unauthorized transfers. (However, refer to comment 2(m)–2 regarding termination of the authority of given by the consumer to another person.)
My Opinions Only