Since you used the term "billing error" I am basing my response on this being a credit card subject to Regulation Z.
1026.13(a)(3) includes the following definition of a billing error:
A reflection on or with a periodic statement of an extension of credit for property or services not accepted by the consumer or the consumer's designee, or not delivered to the consumer or the consumer's designee as agreed. However, as you note, the consumer has to provide a notice of billing error within 60 days of billing statement. From a Reg Z perspective, you deny the claim because the consumer did not meet the timing requirements.
However, from a Mastercard perspective, you have a cotractual obligation to assist the cardholder. The "Services not rendered: chargeback reason can be filed up to 120 days after the promised completion date of the services, providing that it does not exceed 540 days from the transaction date. Under Mastercard rules you should accept the customer claim and attempt the chargeback on their behalf. Since Reg Z does not apply, you don't have to worry about its resolution timeframes and can await the outcome of the chargeback process before making and adjustments to the account.
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