Editing page content through FTP?

If the content is dynamically written, you won’t be able to change this through ftp.
At the template files the content is called by typing <?php the_content(); ?>(or the_content(); ) if already scripting in php.

You need to get his WordPress login information to log in to his website, over there you can see pages, and the names of the pages, once in there it is really easy to edit.
The menu structure will mostly be found in appearence > menu (also in wordpress itself, not ftp)

if you see get_header(); it calls the file in that directory called header.php, same with sidebar etc..

Sometimes there is a little html in it, sometimes it is all dynamicly.