Update
Well I went to the ticket counter here in HEL and showed the card used for payment. They confirmed what the phone rep. had said that there was a special remark for check-in to ask for the credit card. Funny thing was that they couldn't see the actual number of the credit card in the reservation... Anyway they removed the remark so my friends should be ok to travel. I called NWA again and the agent insisted the card does not need to be shown at check-in ("that is never required Sir"), so it seems like the remark has been removed. I'll post a follow up here in April if there are any troubles..