The WebAssist tools do not allow for updating the database using a CSV.
you would need to use phpMyAdmin to update the database in this way.
as for my opinion of allowing the client to do this, it would depend entirely on the client. I would need to take into account their knowledge of the database structure and abilities.
while it is convenient to update the database en mass using a CSV, it is also a bit tricky. it is quite easy to create a CSV file that will fail halfway through, and there is no way to undo the update other than revert to a backup.
I would train the client on backing up the database. I would also create a local copy of it to use for testing the import to work out any problems, have the client use the local copy of the database until they are sure the CSV is properly formatted before they import it to the live database.