I was diving with my grandson and his mother, and we were in a delta formation with me in front. We were on a typical south Florida reef. When it came time to end the dive, I turned to them and gave the signal. They were side by side, about 5 feet apart. When I signaled, my grandson rose about 5-6 feet as he prepared for the ascent, then looked back for his mother. At the same time, his mother reached for her console and took a look at it. She put it back and then looked around. There was no one to be seen. Her son had been there a few seconds ago, and then he was gone! She was in immediate terror. I know that because her son and I were both only a few feet away, about 5-6 feet higher than her in the water column. She did violent turns looking for us, and I dropped down so she could see me. We had a good laugh about it later.
What if one of us had suddenly lost buoyancy control in those few seconds? She would have had no idea where we were, and I cannot begin to believe it would have been her fault.
That is roughly what happened in this case. The guy looked away for a few seconds, and the victim had an uncontrolled ascent. He did manage to see her go up, but it was too late for him to do anything.
This sort of thing can happen on any dive.