Anthony, I have no idea 'why' this is happening, but a twbx file is really just a ZIP file by another name. So if he collects all the files (including the folder structure) into any zip file and then renames it .twbx it should run in any Tableau program including Reader.
Your sharepoint admin needs to add twbx as mime filetype in sharepoint. (sorry, don't have the exact steps, but you should be able to find it searching for 'add filetype to sharepoint' or similar.) We ran into a similar situation when we started dishing up twbx files via SharePoint and Apache at work. Similar thing needs to be done with Apache. If the http server doesn't specify the filetype when the user downloads a file, some browsers will look at the beginning of the file to determine the filetype, and twbx files are essentially zip files with the extension changed to twbx.
In this case, it sounds like your user's browser may be unzipping the files automatically. If not, and he's just navigating into the zip file without realizing (easy to do if file extensions are hidden), the file can just be renamed from yourworkbook.zip to yourworkbook.twbx, and should open with Tableau reader or desktop as noted in the previous reply.
(typed on my smartphone, pls excuse typos)