When in family sharing only the Family Organizer's Credit card is billed.
She can however, buy iTunes Gift cards, and redeem those to her account. Her purhcases will then use the Gift card balance as long as its enough to cover the purchase.
If its not, it will charge the remainder to the Family organizer's card.
Family purchases and payments - Apple Support
With Family Sharing, one adult in the family—the family organizer—agrees to pay for any iTunes, iBooks, and App Store purchases that other family members initiate while they're part of the family group. This also includes content that can't be shared with the family, such as ongoing subscription purchases initiated by family members, including iCloud storage, and iTunes Match subscriptions.
Making purchases
After you set up your family, any time a family member initiates a new purchase it will be billed directly to your account, unless that family member has gift or store credit. First, their store credit will be used to pay the partial or total bill. The remainder will bill to the family organizer's card.