If, 'modified by a specific user' is equivalent to the file's 'owner' as recorded by the operating system, you could use something as simple as:
DIR C:\SomeFolder /Q > FileList.txt
from a command prompt, then open & sort the entries in FileList.txt (e.g. using Excel), where 'C:\SomeFolder' specifies both the drive and folder to process. Otherwise, you'll need a script that can interrogate each file's metadata. You could also add the /S switch to process sub-folders of the specified folder, as in:
DIR C:\SomeFolder /Q /S > FileList.txt

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]