I'm asking questions that need to be answered in light of this incident. I would definitely blame myself if something like this were to occur. Direct eye contact is not necessary but being able to see the students is.
Around here we are lucky to have 20 feet of vis. 10-15 is often more the case. So rather than lead by staying ahead of students I will be beside them, over them, or sometimes behind them. Navigating the quarry, lake, etc is not just my task but the students as well. My policy is no more than two students at a time on the first two dives.
Now granted I keep my classes small, but it can be done and still get 6 divers a day through on the first dives. Means a lot of bottom time for me but more than doable. As a DM I more than once did 8 dives a day.
When doing this I take two students who are always in proper buddy position. Ie no more than a 1/4 turn of the head to see each other. Poor vis and then touch contact is used. Depending on conditions, and how they do day one, I decide if the group can go together on 3 and 4 the next day. If they can then we do. But if there is any question I might lose sight of all of them we do the buddy pair per dive only. Or I make sure I have an assistant. Last time I had 4 OW students on a dive at the same time I had an assistant and two other highly experienced divers tagging along watching over their kids and one guys wife who was diving with her dad. We adjusted the pace to keep the group together and I had the students lead the dives after giving them instructions on where to go. This allowed me to move around all of them and not lose sight of any of them.
And like you I want to know where her buddy was. And was he even aware of what he was supposed to do as a buddy? This is perhaps the most distressing thing. I cannot think of a worse feeling or realization. And if he was not aware of what was happening why not? I looked at this in the presentation I developed on the "Failure of the Buddy System" that I worked on after the rather spectacular thread about the death in Grand Cayman back in 09. In that case and in the fatailites that I looked at for the article it came down to one glaring fact.
The buddy system works!
IF people are taught how to use it. Among new divers that was the overwhelming thing that came out. Though the system was talked about in the classes it was never really put into practice. Students were not paired up and made responsible for each other. They did not have sufficient time to practice it. And by removing rescue skills from the OW class and not looking at how quickly this activity can kill, the real value of the buddy system was not driven home. Then students were led around on tours single file further eroding the lesson of the buddy system. It was just like saying "this is what you should do but we are not going to so that shows you how important it is."
So where is the lesson? And what is it? This is why I buddy students up from day one. Before they even get on scuba it is explained that at no time do you leave or lose sight of your buddy. During the swim tests they are required to stay together. No racing. When treading I like them within reach of each other just to reinforce the lesson. When doing skills they do them one at a time but the buddy stays within reach. Doing air shares I want the buddy next to them with their octo ready to hand off. By the third or fourth session they do not even have to be told where to be. They just move there. They help each other with gear, are constantly comparing gauges, giving the ok sign, and developing the situational awareness they will need in Open Water.
I can't do this overnight in a quickie course. People say students are too busy worrying about themselves to really keep an eye on their buddy during checkouts. If this is true then they are not ready to be on these dives. IF they are that nervous and apprhensive that they cannot maintain good buddy procedures then they should not be doing the dives. It's just a parroting back of the bare minimums and hoping to come out alive.
It's why SEI has a 12 - 16 hour minimum for pool training standard. With the higher end being strongly recommended. And why we still teach rescue skills. To ingrain the lessons and make sure that the student understands the importance of them. It has a chance to sink in. A 4 - 6 hour pool course is not sufficient for that to happen with many people in my experience and based on what I see at the dive sites I have been to. Oh they get through the skills but do they understand them and more importantly will they retain them a month later?