You could probably go the other direction where you import the tracks from your old library into your partners library (File->Add Folder to Library) and that would keep the play counts for your current music and lose the play counts for music tat was in your library.
The other possibility is to write code to export play counts from her library (create list of track names with play counts in a text file from selected tracks), import the tracks into your library, and import the play counts to your library (read list from file, find track in your library and update play counts). The code would not be too hard. I just don't have the time to do it right now.
Have you backed up your iTunes Library this week? If not, you are risking losing your music and playlists....