The files are way too small to contain her notebooks (largest file in the dir is Preferences.dat at whopping 47KB). The only local OneNote files that I have been able to find on the Mac are located at ~/Library/Containers//Data/Library/Application Support/Microsoft User Data/OneNote/15.0/, but there's nothing in there other than some. If you launch OneNote and do not sign in, you will be presented with the screen in the attached image when you try to create a new notebook and cannot move forward until you sign in with a valid OneDrive account.

It appears OneNote 2016 for Mac (we are using version 15.33) does not allow the creation of local notebooks.