In real life would his friends be doing the investigation and autopsy? I would think that they would try to get someone who did not know him as well but of course that wouldn't make for as interesting of a show.
Rory Cochrane left the show to persue his movie career with more focus- he started off in the movies (Dazed and Confused and Empire Records). He is being replaced by an actor named Jonathan Togo- link to his pic here: http://www.imdb.com/name/nm1057047/Jonathan will make his first appearance this Monday so it will be interesting to see how his character fits in with the rest of the characters.
I'm a CSI (all of them) junkie. LOL