I've been searching around on the forum and google for a couple of hours, but most of the mfgtool info seems to be written for kindle devices.
Found one thread with a download for the M92:
https://www.mobileread.com/forums/sho...d.php?t=235195
Unfortunately my windows 8 pc is unable to find the correct drivers for the M91S, so it won't show up in mfgtool.
My linux pc does recognize the device when in download mode, but there's no mfgtool for linux, only imx_usb_loader, a command line tool that can directly flash binary files. I wouldn't know what file to flash.
I'm affraid i'll just have to give up and tell the owner to buy a new e-reader.
Thanks a lot for your help though!