There are site viruses that serve up normal content to normal users, and so go undetected, but serve up spam like that only to GoogleBot, since they're designed to affect PageRank etc. of specific sites.

That was likely the case.

It might have been advertisements, and Google didn't save style information, so it's a lot more visible in the cached version than it was in the live version.

The guy said the site was "on autopilot" for 18 months, so chances are that it was hacked and spammed.