Which phone model does he have? Have you looked at the data usage on the phone itself to see what apps are using the data? Just because how the person using the phone doesn't change doesn't mean that the applications have changed how they are using the data. Settings could have changed to allow the cellular data to be used when it was previously set to use wifi only.
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Verizon is under the gun because they have 10 times the usual reports of unreasonable and unsubstantiated data overage. While it's still the least likely reason for overage the answer is still on your phone.
Look in settings, data usage and see what apps used the most amount of data.
Make sure the phone is connecting to wifi.
Help is here! A large spike in data usage is definitely concerning. Please allow me to help look into this matter for you. I’m going to send you a Private Message. Please reply to the Private Message so that I can gather much needed information to assist you. Thanks!
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New feature added in a software update on my phone (iPhone) called Wifi Assist. This uses data from plan if the Wifi signal is weak. I have Fios and yet it used up all my data really fast. I turned off the Wifi Assist feature in the setting to not allow this to continue. It seems Verizon would mention this but I haven't seen it yet in anyone's posts. Also you can set the phone to 3G instead of the 4G LTE which uses up data faster also. Turn off apps not being used regularly. There is a setting in the phone that could be on which allows for automatic updates on apps and this will use data, even while your sleeping. I hope this helps.
Wifi assist is not new. It was added with iOS 9 in September 2015.
I leave it on on my iPad and never had much data use at all.