IPhone – iTunes: How to transfer a music purchase between two Apple IDs

It is a tiny bit ugly - but there are formal steps to accomplish exactly what you wish. Home Sharing doesn't work so well when both accounts have completely separate libraries - each in the usual home folder - so that's not a good thing to try.

The best workaround is to agree to store the media in a shared folder. It's more work to add the same music to both libraries - but you both have totally independent backups, playlists, play counts, ratings and apps.

This works really well and you can easily recombine later or split her or your account to another mac or PC any time you choose by taking a copy of the reference files as well as the library organization files.

Plug her phone back into iTunes. In iTunes, click on her iphone, and in Options uncheck anything having to do with "Syncing Automatically". If it's not already checked, click "Manually manage music and videos".

If you want her to have a separate iTunes library, she should also have a separate login for the shared computer. This is really the easiest way to deal anything on a shared computer.

Incidentally, there shouldn't really be any reason to physically plug your iPhone into your computer anymore. You can actually set the phone (and iPads, etc) to sync wirelessly.