You would need to contact support. 1-800-4my-xbox if you're in the US/Canada or
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It sounds like the code wasn't redeemed but instead content was purchased. Unfortunately, there are no refunds unless it was indeed an error when redeeming the code.
it also depends on the payment option.
if he added points he would have a currency credit on his ms account which he would select as payment when completing the transaction.
he may have selected your card details by error when making the purchase instead of using the ms account balance .