Having been at Breakwater many times, including attempting a night dive (had to thumb it because vis was less than a foot), I know how easy it is to lose sight of someone. The bottom gets stirred up so easily, especially after a day of diving, visibility is often only 3-5 feet on a good day. You have to swim through kelp usually to get yourself back into shore, the first report was that he was wrapped up in kelp, not sure if that's correct or not, but extremely possible.
I also know first hand that if it were possible to save this poor soul, the folks out at CHOMP (Community Hospital of Monterey Peninsula) would have been the ones to do it. Excellent facility, and they have unfortunately a lot of experience with injured divers.
To the divers family: I am very sorry for your loss. Know that he died doing something he loved and is in a better place now.
To the DM/Instructor: It's not your fault, you did what you could, no one blames you, we all take risks when we dive and we accept those risks. It sounds like you found help ASAP, you did good.