Lou,
The PHP files are not editable for security and copyright reasons, so it wouldn't be possible to modify them yourself (or by your client).
The only way I can think of that may let you do this would be a "syncronization" between both - that is, to place your ActiveDirectory users into PrinShop Web's database (which is, after all, a MySQL database that's writable from outside of PSW). However this is dependant on your ability to write a script or program to do this, as well as the requirement of having access to the passwords in AD...
Perhaps this can be put as a feature request for a later version of PrintShop Web. Alternitively, if this is critical to your customer, our developers may be able to write a modified PHP for you, but that would be a paid professional service.
Regards,
Eric