15+ files really does not make a difference, it is what is inside those files. If you are just searching on text/number/date fields, it is doubtful you can run up much bandwidth. However, if you're accessing PDF's, images, and particularly video files, your bandwidth can get sucked up big time.
You can always go to Settings > General > Usage to monitor your usage, but that is for the whole iPad and not just FileMaker Go. What you want is an app that separately monitors each app's bandwidth usage. That cannot be done by a regular app, it would need to be a service or at the OS level. Since Apple sandboxes Apps, it is doubtful anything like this exists unless Apple decides to include it with the OS someday.
I would investigate their iPad usage for things like FaceTime, YouTube, pictures saved and synced to the iCloud, GPSs apps that are continually running as being much more likely to errode your data plan than FileMaker Go.
My suggestion would be to have them spend a month with the iPad and turn off all other apps and only use FM Go and watch the bandwidth in the Settings > General > Usage.
Thanks. I've been doing some additional research as well and discovered that you can go to Settings > Cellular > Use Cellular Data For and there it will list all the apps that can use cellular data. It also allows you to see how much cellular data each specific app has used. This will allow me to check with the users to see if FileMaker is really using the amount of data they think or if it's something else.
I have a feeling its really something else, but this will allow me to confirm.