Aside from the obvious issues that people have stated above about how well dive ops and DMs make people aware of the hazards and the limitations of the DM in protecting them from harm, I have realized another thing in reading about this incident. This is not to place any blame whatsoever on Cristina's husband, because prior to this incident, I would have done exactly what he did. What I realized is that at the point my buddy notifies me of a problem, especially one they want to abort the dive to address, I need to either maintain physical contact or keep them in my sight at all times after that so that I can be ready to help if needed. I already carry a noisemaker in my hand; a metal locking carabiner that I can either click the gate on to get my buddy's attention or can bang against my tank to get everybody's attention. So in the future, if I find myself in this kind of situation, when my buddy says they need to abort the dive, I will grab him or her by the BC with one hand and bang on my tank with the other to get the DM's attention. Then we can ascend together.
I had an incident in Belize many years ago that is actually somewhat similar to what happened here. My wife and I were fairly new divers (~30 dives or so). The winds were blowing onshore at 15-25 kts with higher gusts. Waves were breaking onto the reef in the 4-6' range with a few higher swells approaching 8'. For the morning dive, we stayed inside the reef. Our lunch was very late in coming due to slow service and I wolfed down a burger, then ran to the boat for the afternoon dive, thinking we would be inside the reef again. When the boat crew requested a vote on diving outside the reef, only my wife and I voted to stay inside. I should have called the dive then, because I get seasick very easily and wasn't comfortable with drift diving in such rough conditions. But I didn't want to get stuck waiting on the boat in those seas either. So we did the dive. I tried to stay shallower knowing that I used air faster and didn't want to sit in the boat waiting for the rest of the divers. What I also did was subject myself to more surge action than the rest of the divers, and I got seasick.
One of two smart moves I made was to go to my wife and grab onto her BC. Then I did my second smart move. Using skills I learned in Rescue Diver class (that we took right before going on the trip), I vomited at 80 feet underwater 3 times and each time, I made sure to push the purge button on my reg before I stuck it back in my mouth so I wouldn't aspirate particles. Nobody else on the dive knew what was happening until they saw the fish eagerly helping consume my little chum clouds. Looking back, the main reason I grabbed my wife was to secure my buoyancy. She had no idea what was coming, and when I kept turning away from her to try to puke, she kept turning me back to her. By grabbing on to her, I established a connection and avoided losing my buddy or the group.
I realize now that had I tried to puke by myself, not only would I have risked losing buoyancy control, but I could have also blacked out, ingested water, lost contact with the group, or worse. If I had panicked and ascended early, we would have forced the boat to leave the group's bubbles to pick us up, and at the time, we had no SMBs (we do now). So looking back, I did a dive I shouldn't have done, and wouldn't do now. We got a very limited brief on procedures from the DM and boat captain. And the best move I made all day was to grab my buddy and not let go when I got into a difficult situation. It is a shame it took a thread about such a tragic incident to remind me what a great lesson I learned way back when.