Hi Nick,
The system did change a while back because of GDPR concerns over the forms used in shops being returned to the central office. It’s now not possible to record registration on the card unless it is done when the card is purchased online and sent to you.
I’m fairly certain that the process of adding the card to an account is now the same as registering it, so I wonder if the card was actually registered before. I’m very certain that you can only attach a card to one account, so yes it will be a problem if she’s trying to add it to hers.
I think the best course of action here is to contact the helpdesk and ask them to double check everything. They may need you to be there at the same time if the card has to be uncoupled from your account. I hope they manage to sort it out.