Facebook, YouTube, iMessage (sending lots of photos. They don't compress), other apps that use data. Also she may have wifi assist turned on and that will leverage wifi and cellular for faster and more reliable download speed. Without knowing her usage hard to tell how to limit her.
CBeach73, if you go to Settings > Cellular - and scroll down to Use Cellular Data For: It should should how much data each App is using. Just below that you can Reset Statistics which will clear it out. So you can get a newer reading as the iPhone is being used.