Don;
This is a transfer by ownership, assuming the transfer was done with a Google transfer. It should move the folder created by the transfer, and its subfolders, to the root directory of the original owner's MyDrive. You will still need to rearrange any subdirectories and files to move them back to their proper locations, and any permissions/sharing will likely have been altered or removed in the initial transfer.
$> gam user <current user> transfer ownership <folderID to move> user <original user>
NOTE: what you are about to do is fraught with peril. I would recommend you try some experiments beforehand. For instance create a folder on the source user's MyDrive and populate it with a file or two, then try this out. You can then decide whether to proceed with the big one. I'm sure Ross will have either comments or corrections to my suggestion.
-Marsh