She should not need to cancel her own subscription directly. If she has accepted the invitation to your Family Sharing group, her subscription will be cancelled automatically by the system the next time its set to renew, and will start using the shared subscription instead.
When this happens, all her music is preserved.
How exactly did you "add her to yours"?
click here ➜ Add a family member to your shared subscriptions - Apple Support
and here ➜ Get an Apple Music family subscription - Apple Support
If someone in the family group already has an individual or student
subscription to Apple Music, renewal of that subscription is
automatically canceled when they join the family group.