This is one reason im a bit apprehensive about doing my DM. The guy was certified. He should be the one that is responsible for his own actions. Yeah it does suck that he died but shouldnt he be the one saying he wasnt ok to do the dive? Shouldnt he of been the one to check his weights? Passing on from all that could the said dive shop now couter sue the cert agency and instucting shop for failing to teach the guy to the standard?
Condolences to the family and loved ones for their loss but really the only one responsible for the actions is yourself.

Matt