Great ideas. Thanks.

I'm going to
1) Have him leave his laptop on overnight, connected to internet, to continue to sync between laptop and OneDrive.
2) Download everything from OneDrive via my laptop connection to his OneDrive to my computer.
3) Give the abovementioned utility a try to try and shut down OneDrive (and, as part of the process) bring his One Drive files back to his computer
4) Upload a false dummy file to his OneDrive to keep it "full" and maybe prevent him from doing this - by trickery, sorcery, accident, or otherwise - in the future.

Any other thoughts appreciated.

Will advise.