If you are jailbroken, you can passcode the settings application for your own protection. If not jailbroken and you do not want to mess with that, I'm not sure what to tell you. What is she using your phone for? Do you not want a passcode on it for your own protection?
As you said, you can turn the passcode off and not require one. You can also set when to require it, for example 'After 1 minute' or as much as 'After 4 hours'.