I find that CMS made simple is quite user friendly. I don't remember if it's got WYSIWYG built in, but you can get FCKEditor easily enough. It's got a no-download/upload module management system, so you can simply browse a list of currently available modules from their repository (alphabetical), and hit "download and install". Within a few seconds, it's done. I would advise your client to not do a lot with modules himself, because if you install a module without dependancies (and it lets you) your installation will get bugged.
He'll probably want his same layout too, so either you or some company will have to convert that to work with CMS made simple.