It is ultimately just a matter of restoring the CSS file for PowerCMS Admin panel. I would try the following if I were in your situation. (NOTE: I assume that you've saved your CSS menu as a preset for easy restoration. If not, you should always save your new menu as a preset so you could easily restore an identical version in the future.)
1) Create a new folder on your computer and then create a new site in Dreamweaver.
2) Create a blank HTML file in the new site, type in a few lines of sentence as a placeholder and save the HTML file.
3) Run PowerCMS. Convert the placeholder text into a PowerCMS region. Save all, and run the setup wizard to set up PowerCMS. This should create a new PowerCMS package with its own "cssmw" directory.
4) Go back to your client's site in Dreamweaver, go to the site management window (File window), and rename "cssmw" (inside "CSSMenuWriter" folder) to "cssmw0". This should trigger Dreamweaver to update all CSS links in the site. If it doesn't, you can recreate your client's menu in the placeholder site AFTER creating PowerCMS package for it, or use a decent text editor and run multi-file search and replace so all CSS Menu Writer CSS links would point to "cssmw0" folder.
5) Copy the new "cssmw" folder (and "cssmw0" folder as well if you had to resort to creating a new CSS Menu) to the corresponding location in the client's site. (It's in "CSSMenuWriter" folder.) Then, change the stylesheet linking for CSS menu in your client's site so all page would look for "cssmw0" to render its site menu.
The point here is that the admin panel and its related files for PowerCMS are always the same. So you can just regenerate the CSS files for PowerCMS in a fake site and restore the deleted one this way.
Hope this helps. It worked for me in my case.