She needs to sign in on her device(s) with the same Apple ID as used for the invite. If the new Apple ID was registered after the invite was send, then cancel that and invite her again with the new Apple ID.
If the device was previously signed in with a different Apple ID, then that has to be signed our first (Settings﹥[your name]﹥Sign Out). Then sign in again.
The device should have its last association with a previous Apple ID more than 90 days ago.
If she is just now using a device that was previously used by someone else, then it may be best to reset that device, and configure for the new user with the new Apple ID.
If you still can’t see or use Apple Music and the iCloud Music Library:
- Restart your iOS device. Now try again.
- Turn off iCloud Music Library and then turn it back on. Now try again.
- Sign out of the iTunes Store and then sign back in. Now try again.