Prove that he's illegally downloading them...
Anyway, if your ISP is like mine, it will restrict DL speeds for certain times of the day. I get a maximum of 5KB/s up until 12am when they release the chains and allow the full 40Mb potential, up until 10am
If he's using torrents to download a season of a tv show, he's illegally downloading it since no company releases torrents of their shows legitimately. I'm not making a judgement call here, I'm simply stating that if it's a broadcast tv show, there's no legal way to download them using torrents.
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Since there are no viruses for Mac OS X in public circulation at this time, it is safe to look at "pirated" porn, one should stay away from pirated applications though, as they can contain other kinds of malware.
Sorry, didn't mean to make the implication that you would get a virus from the torrent on a mac, I'm assuming the OPs fears are from doing this on a PC, likely getting a virus from one of those two options.