I am not sure about this, but I think if you try to redeem a valid code, it will ask you if you are sure to redeem it or confirm the redemption. You can then cancel and be sure your Card is not redeemed. Again am not sure about the confirmation, but anyone who has done this should be able to assist us more.So you mean, after you redeemed it he will redeem it a second time to get an error and send you this screenshot? Ok he shall send a screenshot showing his account, then you will see your redeemed code.
Further, how could you test it before sending it? If you redeem it, then it also has already been redeemed on your own account, so you cannot send it to him any longer?!