iPhone 6: How to interrupt software updates that continue to use mobile data?
Fischer, there are two different types of software updates that occur on iOS; updates to the apps you have installed and updates to the operating system itself. 

    To disable app updates over your friend’s cellular connection, navigate to Settings → iTunes & App Store and scroll to the very bottom. Tap the toggle next to “Use Cellular Data,” but keep in mind that this will also disable the use of iTunes Match and iTunes Radio over the phone’s cellular connection.

   If you are concerned about updates to iOS itself being installed over your friend’s cellular connection, don’t be — having contacted a friend of mine who is an Apple Genius, she tells me that your iPhone won’t install updates to your operating system without a connection to WiFi or the user’s specific permission.

   If your friend continues to have an issue, she may want to contact Apple about a potential bug.