I have tried to read all the posts on this thread, but I got to about yesterday lunch time, and I had to quit. I am going to post now.
First of all, my condolences to you, Fosterboxermom and the diver's family and especially his fiance. Even though it seems like you didn't know the deceased diver very long, you have stood up admirably for the defense of their cause. No one reading these posts would not want you on their side if an unfortunate accident happened to one of us. However, it appears (based on the posts) that negligence in this case is at all levels. Of course this is just my opinion. That's all anything is on these forums anyway.
The dive op should have organized a trip for advanced divers separately from basic divers; that should have been made clear at booking. That probably rarely happens, but I have seen DMs on boats checking for advanced certs before moving to a site. That said, novice divers on their 7th dive have successfully dove to 100 feet. I just checked my log book, I made my first dive to 100 feet at dive #10 during AOW class at Looe Key. I never went below 60 feet as an OW diver, but wanted to move to AOW quickly. I always valued my training, and still do. Foster, you stated that the training was completed the day before. Was the instructor on the dive? Was the instruction a standard PADI course, or the accelerated tourist variety?
Caymans is clear open water. The DM should have theoretically been able to track divers at 60 and 100 feet, but why lead a group of novice divers in this way? The DM should have made the call at the dive site to lead all the divers together at 60 feet when he was told about the experience inadequacies in the group he was leading. If you have lemons, make lemonade.
And I have to say... Brendan made a poor choice in separating from his fiance. He should have tucked his ego aside and just done the minimum at his experience level. He should have thought to himself... my fiance is uncomfortable diving at 100 feet, but what if she has a problem at 60 feet? I want to be there for her... he should have foregone that depth and stayed on the cautious side of depth on a seventh dive.
Divemasters lead dives, but the better ones do all the things that I have read other people mention in this thread... they check air, they tighten straps, they make minor repairs to equipment, and do everything they can to help ensure the customers have a safe, pleasant diving experience and come back again. They always explain the site to me. I almost always have questions about current and depth and where are all the assist lines? In all my diving in the Caymans, this was never an issue because of the clarity of the water. If the DM in this case did not do little things to assist on board, it may be poor business practice, but is probably not criminal negligence. It is still the diver's responsibility to know and understand the site. But OW divers are taught to rely on their buddy... maybe that needs some evaluation. Who was Brendan's buddy?
All of my experience in the Caymans has been on drift dives, so I am unfamiliar with the practice of a captain abandoning a moored or anchored vessel, changing over to be the divemaster, and then returning to the boat and captaining the boat back to shore. Come to think of it, it couldn't be moored... how could the captain steer and tie in the mooring line? So they are dropping anchor on the reef or wall or what? Hmmm. However, if there is a law that allows this practice, I would certainly not dive with a dive op that supports it. Just because something is legal does not necessarily mean that it is the right thing to do. I would dive with an operation that, as a minimum, has a captain and a divemaster. How would a novice diver know that this is a problem? They have no experience to tell them this is an issue. Take a lesson that this is something that you do not want to walk into on your next trip to the Caymans (or anywhere else). However, if you are diving off the wall at Sunset House, you may not have anyone but you or you and your buddy to look after each other anyway. As others have said, it is ultimately the diver's responsibility to know their limitations and dive safely within them.
Should the Cayman Islands have changed their law (if there is such a thing) to allow no observer on a boat dive? Maybe there is some blame to go around for everyone on this one.
Personally, the Caymans is one of my favorite places in the whole world. It is such a shame that this sort of thing happens needlessly there (or anywhere else new OW certs roam).
PADI, you need to open your eyes and stop thinking about more qualifications for basic OW certs and put them in place. There is too much to know and not enough taught at that most important entry level. It can't be all about making money all the time. As divers, if we don't see these kinds of incidents reduced, we will see our sport regulated one day. It is up to all of us to assist new divers that we see. No it is not an obligation and you are not responsible for helping a new diver. It's just the right thing to do. Remember, someone helped you once.