Wonder if its a glitch with your iCloud back up. When you connect to a power source and wifi it may be backing up but using your data instead of the wifi. I would have Apple do a diagnostic on your device to see.
I understand that. When you view the data on the website it specifically says when the data was burned, there are a dozen pages with a few kb's to 5 MB's and then the two anomolys. 430 AM and 230 Pm ish the next day with massive amounts used. I typically use about 800 mb's to 1.2 gb's a month and 6 days into the billing cycle I had already used 1.3 gb's. This was something either with the phone continuously updating or error in reporting. I can go back to when I got my 4s and the data usage is fairly consistent.
Check the settings for the Facebook app, if she has it installed. This app now preloads and plays videos in your feed/stream/whatever they call it.
The settings are in the iOS settings app, under Facebook, and not within the Facebook app itself.
Could be the culprit. Also, double check that she is on wifi at home. I know so many people (parents included) that have issues only to realize they are not connected to their home wifi. I have no idea how this happens, because it's after I've checked they were set to automatically connect, but it does. Also know of a firsthand situation where the home wifi signal wasn't strong enough to cross the entire house, and they didn't realize it. So again, double/triple-check wifi connectivity throughout the house.
Yes, the Facebook app, I turned off auto streaming videos on my Note 3 and data utilization went way down, darn you Facebook!