<fastethernet got there quicker but Ill post anyway!>

It is my understanding that Karoo are acting on the behalf of the copyright owner (or their agent) when sending you a letter about disconnection.
What the copyright owner does is seed a copy of Sims 2 themselves over bittorent (or more usually through a company they hire for this purpose) and when you connect to download (or perhaps try to upload to them) the copyright owner sends Karoo an email telling them something like:
"@ 19:23 GMT on 15th December 2008 IP address 218.32.22.3 illegally uploaded/downloaded a copy of Sims 2 with our server".
Thats when the law is broken.
Viewing a torrent on a website does not constitute breaking the law because the torrent file itself does not contain any copyrighted material.
I suggest that your friend (perhaps accidently) downloaded the Sims 2 because otherwise the copyright owner would not know about it. The copyright owner does not have access to the Mininova web access logs, and I doubt if Karoo can be bothered to police every person who visits the Minninova network. Even if they could be bothered there would be no point as your friend never broke the law.