If you only want them to modify the content portion of the page, consider using a PHP "include" in your page to include their separate file for the content. They could modify the content file using any editor they wanted and upload the content file using FTP. Then your page would include their content file into the page.
* You would need to rename your page to have a .php file extension.
* Then in your page include the file with this code.
<?php
$file="http://yourdomain.com/includedpagename.htm";
include ($file);
?>
* The included page could even be a text file with a .txt extension.
* Their content file can even be on a separate web site, such as the web site space furnished to them by their ISP provider.
* If you want the content file to be on your web site but don't want the content author to have access to all of the files, create a subfolder and place their content file in the subfolder. Then use the LunarPages cPanel control panel to create an FTP user with rights only in that folder. In that case your include code would include the folder name in the path.
<?php
$file="http://yourdomain.com/foldername/includedpagename.htm";
include ($file);
?>