The only way for her to use her own card, is if you turn off Purchase Sharing in the Family Sharing settings.
With that, she won't have any access to your purchased apps, movies, tv shows or music and will be charged on whatever payment method has been added to her Apple ID directly, but can still access the shared Apple subscriptions such as Apple Music and iCloud Storage.
click here ➜ Turn off purchase sharing and stop sharing a payment method - Apple Support
Do note, this change affects everyone on the family group. So everyone will need to use their own payment method to make purchases once you turn it off.