I've used both extensively. I've also done some Ruby and am exploring RoR.
I use Perl hands down when I want to do anything complicated. I'd ask him if he ever will want to create any charts in is site, or talk to another system, or feed users Excel spreadsheets on command.
PHP has lots of plusses -- it definitely easier, and, yes, I mix code and HTML -- but the module collection on CPAN brings me back to Perl every time.
Ask him if he wants to make money from his web site now or "in the future". :D