I have to say that I'm quite impressed with how easy it actually is.
My html/php and css skills are from about a decade ago but editing templates 'feels' right to me; it's how I used to layout my sites
The design/layout I'm going to create is never going to be as professional as it would be if he paid a web developer to do it but hopefully it'll help to show him what's possible.
It took about 2 hours to install/configure WP and then port his existing site into a WP format ! Now the hard work of improving the site begins