If a phone has anything running, any app, even when you aren't actively using it, then it can be using data. Say you have facebook running, even in the background and you aren't 'doing' anything on it.
To make sure it isn't using any data at all, you or she would have to make sure that all apps are closed and turned off. Any app that accesses the internet. Then turn them off when using and again turn them off when not using. You may have to check the settings for the app as well, because some are set to turn on and start running the moment the phone turns on.