In my industry, the #1 site has disappeared and he is having to pay (a lot) for Adwords. Google showed about 36,000 inbound links to his site and most of them came from 2 sites, thousands of links from a couple of large US-based sites, linked from every page. I reckon he was paying. The industry average for Google-recognised inbound links is between 50-400 so he did stand out as having rather a lot!
I can't see any other problems with his site, so I reckon it was his link programme that did it. However, the bit I don't understand, is that Google still seems to recognise his 36000 links - I thought they would have ignored them if they had penalised him for it