I suck at PHP and I could probably do that. It's not very hard to do. You just need to create an admin area for your client (using htaccess would be best here since it's only one account). where he will be able to create accounts for his clients using a mysql databases. Once again this is incredibly simple to do.
Then you create a client area with a PHP condition checking whether the user is logged on, if (isset($_SESSION['user'] , if that's true, display client area, if not, display login box.
Oh and to answer your initial question, I guess it could be done, but it's not the ideal way to do things. Incorporating Databases into the system will allow you to hold crucial information about your client. Email Address, Number of purchases, you name it. You can't do this with an htaccess file.