If a refund is submitted not long after the original charge occurred, the refund may be processed as a reversal. This means that the original charge will drop off the bank statement altogether, as if it never occurred, and the bank account balance will adjust to credit that money back without a specific line item on the statement.
If you issued a refund and your customer is reporting they have not seen it after ten business days, ask them to look for the original charge on their bank statement. If they do not see the charge on their statement, it was reversed and the refund is complete.
If they still do not see a credit and still see the original charge after ten business days, they should reach out to their bank for more information.