PNO, is the Payment Network Operator. A PNO could be Mastercard, Visa, American Express etc.
The PNO’s all have a similar service that merchants can optionally signup for. Mastercard calls theirs ABU or Automatic Billing Updater. But remember, all PNOs offer the same service. Most bank credit and debit cards are automatically enrolled. The link below gives a good explanation of the service.
https://www.bannerbank.com/financial-resources/automatic-billing-updater
>>What is Mastercard Automatic Billing Updater?
Mastercard’s Automatic Billing Updater is a service participating merchants can use to retrieve cardholder account changes, like new debit or credit card numbers, to ensure as little disruption as possible for your card-on-file transactions and recurring payments.<<
Because the signup your mother did was a subscription (a legal contract) the payment is a card-on-file transaction and the number is being automatically updated and forwarded to the merchant and continue billing your mother.
Get the bank to stop the billing updater and then change the card number and ask the PNO to update the card token in Apple Pay. Your bank will need to send a service ticket to the PNO or TSP (Token Service Provider) to update the token.