Recovering deleted files android 6.0.1 (internal storage) They were onenote's files, research notes. Had problems with sync and lost files.

How can I recover the files in the safest way? It happened today. I already stopped using the phone.

(asking for a friend)
I'd try photorec (testdisk package) in Linux. Or the Piraform file recovery program in Windows. Get it connected to a PC sooner rather than later as you don't want the area it was stored to be overwritten.

Was the Onenote not syncing with a Microsoft account?