Find Your Cruise
Go onto the celebrity website, scroll to the bottom of the page - you will see'gifts'; click that, and there is a section on Cruise Gift Vouchers. Buy online or tel: 1-999-879-7908 and they are sent direct to the recipient.
You can call the Celebrity amenities dept and they can advise what you can do for the couple - 01932 834325.
Not sure how easy this might be, but one way could be to give them a cheque they could cash at the bank on the ship, or before they go but no quite what you are looking for perhaps some one else will come up with a better idea .