He's right you can adjust how often certain apps get to receive data. I would check settings/location services/"any app" just to see what apps are using location services. Depending on the amount of apps on the phone, if all of them are running location services this could cause you issues with data usage. I do believe too that if you change the apps in settings/notifications look at "In Notification Center" this will help with data also. All of the apps listed under "In Notification Center" are pulling data every ten maybe fifteen minutes (I am not sure on the time though) and this could cause data to run high.
I would really check the location services, and how may apps use this. Even if she was connected to a WiFi connection the phone would still use it's data plan to pull the exact location of the phone which is where she is at the time. I am an active user on Google+ so I keep my location services on to help promote small businesses. I have games on my phone that want to use location services, but why would a game like "Cogs" need to know where I am? So I turn it off, and I don't have to worry about it anymore.
Any other questions on this feel free to ask. Hope I was able to help you a little more.
