When it comes to notifying the bank about an unauthorized point-of-sale transaction, can either account owner file the dispute or must it be the account owner whose debit card was used for the unauthorized transaction?

Reg E only states that the notice of error from the consumer should enable the institution to identify the consumer's name and account number and include, if possible, the type, date, and amount of the error. The regulation does not specify which account owner/cardholder can notify the bank of the error. I take this to mean that either account owner can notify the bank, even if it wasn't their card that was used.

Notice can be by anyone, so yes, notice can be provided by either account holder. In the commentary it states:
"2. Notice by third party. Notice to a financial institution by a person acting on the consumer's behalf is considered valid under this section. For example, if a consumer is hospitalized and unable to report the loss or theft of an access device, notice is considered given when someone acting on the consumer's behalf notifies the bank of the loss or theft. A financial institution may require appropriate documentation from the person representing the consumer to establish that the person is acting on the consumer's behalf."

Thank you. This is helpful.

We would allow either account holder to dispute a transaction. However, if the transaction is being claimed as fraudulent, we would want to speak with the cardholder so that the card can be closed and a new card issued.

That's a good point. Thanks!