Apologies. Promised an update.
Got my hands on the laptop.
First, copied over all files from OneDrive (the "local files") back to two separate stand-alone 1TB drives (one for Mr. jsnydcsa Snr. to keep, one for me to keep, in separate locations).
Second, tinkered around with a few settings buried deep in Windows settings to turn off One Drive as a "Recent" file location in Save/Save As menus of Word, Excel, Etc.
Third, copied all files back from one of the stand-alone drives to the laptop's hard drive (e.g. starting at C://)
Fourth, confirmed with user that he could locate the newly moved files and couldn't "find" OneDrive option or filepath for now.
Fifth, erased OneDrive "local files" from laptop hard drive.
Sixth, re-confirmed all files back from on C: drive and nothing in OneDrive on cloud.
Seventh, ran backup software on laptop to backup entire laptop to the aforementioned two separate stand-alone 1TB drives.
Sent Mr. jsnydcsa Snr. back on his way. Happy. But still cursing the evil geniuses of Redmond who "tricked" him in the first to using OneDrive in the first place.

I did try and jigger around with some of the suggested tools above. But, time got away from me and the steps above seemed to solve the problem.

Only about 1 litre of Jameson's consumed during this process.

Onward. Thanks for the help everyone.