You could do a partial refund on one item through Paypal.
Be aware if you do, you will still pay full fees on the original price paid, eBay do not deal with partial refunds. If it was a substantial amount and she agrees, as it was her fault ordering two, you could calculate this fee, and deduct it from the amnount you refund. Your choice.
Go to Paypal account, find transaction, and click on refund this item, be careful to put the correct amount you wish to refund into the little box provided, or you will refund the whole amount.
