Your question is a bit confusing.
How is she using your account for purchases exactly?
If using family sharing, purchases are charged to the Family Organizer, but belong to the Apple Id that initiates the purchase. Assuming her Apple Id is logged in for iTunes & App Store on her iPhone then the purchases she made already belong to her.
If your Apple ID is logged in to iTunes & App Store on her phone, then Family sharing is being entirely bypassed, as your account already has access to its own purchases regardless of device and Family Sharing is not used. In that case all Apps belong to your Apple ID.
If she already has an Apple ID then she already has her own account. The Apple ID is her account.
How exactly are things set up then?
If Family Sharing is correctly used, then she would already have her own account, and her purchases belong to her account / Apple ID, and there should be nothing that needs to be done.