She won't lose her library. She will just not have to pay for it any longer because apparently somebody in your Family Sharing group already has a family level subscription. Each person will have their own personal library and can download items others have purchased unless the other family members decide to hide them.
Add a family member to your shared subscriptions - Apple Support
- You can share many subscriptions from Apple, like Apple TV+ and Apple Fitness+, with your family.
- "When you share subscriptions like Apple Music, Apple Fitness+, or Apple TV+, each person sees their own preferences and recommendations — not the whole family’s. And when you share an iCloud storage plan, family members can't see each others’ photos, files, or documents."
- How to add a family member to your subscription from Apple
1. Open the Settings app.
2. Tap your name, then tap Family Sharing.
3. Tap Set Up Your Family, then follow the onscreen instructions and invite your family members.
Already set up Family Sharing? If yes, use this shortcut to jump to the troubleshooting section of the article --> Add a family member to your shared subscriptions - Apple Support
4. Eligible subscriptions are automatically shared with your family members.
