Hey Chewagirl
I recently helped out a friend who is struggling with their data usage on their cell plan to minimize the impact of the phone's background processes and such from eating his data. Luckily he does not use Facebook, but I can still help!
If she uses the Facebook App for Apple the settings to turn off auto-play videos should be as follows:
Launch the app > on the bottom right click on the "More" option
Scroll down and tap on Settings > Account settings > Videos and Photos > Never autoplay videos (under the "autoplay" option)
Recently we had someone in the community who has an "Apple family" and multiple users using the same Apple ID on multiple devices. If you aren't sure, check with your daughter and other iPhone/iPad users in the house to see if they are aware of their iCloud setttings. If not, chances are everything is set to default and that means every picture, video, soundbite, etc is being backed up when they are on WiFi - possibly even multiple times a day

Hope this helps! I can offer a bit more if needed, but I'm a big Android user so my Apple knowledge is not as extensive in that regard.
Amanda