Hello, my name is ***** ***** I'll try to help you with this query. If he would register for the new google account, then the value of the gift card would not be transferable (if the card number was already accepted). But, it should be possible to regain the access to the old account, as they don't expire. Most likely he just forgot his password, which you (him) can try to reset it via: https://www.google.com/accounts/recovery/
Hello! My name is*****
If you still require assistance, please type the old account address here that was used to redeem a gift card and I will check if there is a way to regain access to that account.