Well the only way to do that is to set the headers using PHP to force a download, but that means you're going to have to put the header information on each page that you want the admin to be able to edit. That might mean putting an IF/ELSE statement on the page that checks the referring URL to see if it's coming from the admin page or not so it doesn't display the headers normally.
http://us3.php.net/header
That might help you out right there, however, why don't you just use fopen() to parse the HTML in a textarea? Oh, wait, nevermind you wanted the admin to edit it in Frontpage.