transfer music

Is the music she has on the iPad in iTunes? ... or are separate audio files? If they are in iTunes, do you both share the same Apple ID? My guess is that would be no. If that's the case, you would need to repurchase any songs from iTunes ... unless you are both on an Apple iCloud Family Sharing plan.

If this is the first time she connects to that iTunes library it will offer to transfer some items but then erase the device. It will only transfer iTunes Store purchases. The idea is that a device is secondary to a computer's iTunes library and anything that was on the device should have originated from a computer or the iTunes Store in the first place. Ideally to get the music that is on her phone you copy the iTunes library that put it there in the first place, or use Home Sharing for Family Sharing. Apple does not explain this policy but you can bet it has to do with anti-piracy measures. The powers that be don't want just anybody transferring device content otherwise anybody could download the entire contents of a device to a friend's computer and hey the party starts.

An i-device was not designed for unique storage of media. It is not a backup device and media transfer was planned with you maintaining a master copy of your media on a computer which is itself independently backed up against loss. To use a device with a different setup you transfer the old library from a computer or a backup directly to the new setup, not the device to the library. Media syncing is one way, computer to device, updating the device content to the content on the computer, not updating or restoring content on a computer from a device. The exceptions are iTunes Store media purchases which can be transferred to a computer.

iTunes for Mac: Transfer iTunes Store purchases from iPod, iPhone, or iPad - https://support.apple.com/kb/PH19615 - This feature works only for media content (not apps) bought from the iTunes Store. Alternatively: Redownload apps, music, movies, TV shows, and books from the iTunes Store, iBooks Store, and App Store - https://support.apple.com/HT201272

For transferring other items from an i-device to a computer you will have to use third party commercial software. See this document by turingtest2: Recovering your iTunes library from your iPod or iOS device - https://discussions.apple.com/docs/DOC-3991 Even this method may not fully recover what you had in the library originally. For example if in order to save space when syncing you had converted music files to a lower bitrate, or photos to a lower resolution, it is those lower quality files you will recover.