No, you cannot deny the dispute based on consumer negligence. The commentary to 1005.6 is very clear that we cannot increase a consumer's liability based on these facts. If the cardholder admits to granting authority to the acquaintance to use the card and they exceeded the authority given, then your customer would be liable. We also do not have to issue a replacement card to this customer.
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.)