An include is a separate file which is "included" into the page via the server.
PHP includes: If you are using static HTML pages you have to put an instruction in the .htaccess file to tell the server to render the PHP. You won't be able to see the inserted file offline, it only works when uploaded to a server. No change required to the existing file extension.
If it was a brand new site you could look at doing server side includes
With the two methods above you would have to teach her how to upload just the include file.
MS FrontPage and Expression Web have design time includes you can use. This way, when you update the include and the set of files for the project the customer would be able to see the included material on their own computer before uploading to the web server.