Here's the screen to add a new card at 21
View attachment 107394
There's a warning that the card must be in your name, and you have to type in the name on the card.
So what name did this person use?
If he used his mother's name (the name which was actually on the card) why would 21's system allow a card to be registered under a different name to the name the casino account is in?
If he used his own name then I'm pretty sure that visa (or whoever) would block the transaction, since the name sent from 21, wouldn't match up with the long number, expiry and CVV code.
I know I once had a transaction refused by Visa, when I'd accidentally made a typo with my name.
So, I'd say it's down to 21, for not cross referencing the declared 'name on the card' against the name the casino account is in. Which basically allows ANY card with ANY name on, to be used to deposit