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If your girlfriend was subscribed to your playlist and you've decided to unpublish it, she would no longer be able to listen to the playlist. However, if she saved a copy of the tracks on the playlist to another playlist, then she will be able to continue listening to it. Be advised though, she will not be able to see any alterations you make to the playlists either so as long as the playlist is unpublished.
Despite the previously stated pieces of information, your girlfriend will continue to be able to see any and all published/non-secret playlists on your profile.
Thats not 100% true, anyone who follows the playlist before you made it secret will still have it tied to their account and therefore it will still appear in their clients so they can listen to the playlist.
If you want to get rid of it, just delete all of the tracks within in and rename it to something meaningless.