I have err99 once when testing a sigma 70-300 on my 400D, seller was kind enough to buy back the lense (bought it without testing).
So far no error on my FF. Your friend must not care much for his cam if he didn't sent it in when getting the first err99 message.
On another note, my friends D200 ate his CF cards - after a few shots, the card just died, put in another card and the same thing happen. **** happens no matter which side you're on.