If the client purchased the theme license, then she can re-access.   If the developer/web designer was the one who purchased the license, and the built the site for the client, then you'll have to get the theme through the developer.

If she did purchase the license, and doesn't remember her download area membership access info, she can reset her password here:

This isn't as much a Market Theme question, as it is a Wordpress question.  (How to protect Wordpress from hacking)

I recommend a couple of security plugins:

1.  Wordpress firewall

2.  Login Lockdown

3.  Secure Wordpress

I also recommend creating an administration user other than the standard "admin" user.  Create another user with admin priviledges, and use that one. 

Another thing is to change the permissions of your "config.php" Wordpress file to "444".

Also, if you can setup your database to use a different prefix other than "wp_", it would be much better.