From: How to share purchases with your family - How to share purchases with your family - Apple Support
"When you turn on purchase sharing, everyone in your family gets access to apps, music, movies, TV shows, and books that family members buy. The family organizer is billed for family members' purchases."
I don't know what you mean when you say "link". If she does not have any account balance then charges automatically roll over to your card as the Organizer.
Some people have reported that they have had to enter a CVV to verify use of the card in this manner but that's all I can think of. Generally she shouldn't have to do anything. If you really think she has to be proactive in this payment method working, contact Apple.
Refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support
Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support