Upon plugging in your device, you may receive a screen that asks whether you want to Setup as New iPhone or Restore from a backup. If the device has already been through the iOS Setup Assistant, Setup as New iPhone only creates the device record on the computer you are plugging it into. This does not remove any content that is already on the device.
After setting it up as a new device on the computer, you will have a selection below Options to Manually Manage Music and Videos. This will allow you to manually select the content you wish to sync to her device. If they are iTunes Store purchases they will be tied to the Apple ID which purchased them.