Detailed information remains to be presented. The statement from Waterhorse is informative, and impressive. I wish that all charter operations / instructional operations would post specific information just as quickly (I also wonder what their insurance carrier and attorney said about the post). Nonethless, it is very much appreciated. The situation as I read it in the OP and subsequent posts is every instructor's nightmare.
I was reading two posts this morning, before seeing this one. The first was from my dive buddy who visited a local quarry Sunday and went to 'The Pit', a 95 foot hole that he and I usually visit every time we go there, which he described THIS TIME as 'spooky' - essentially a blackout. The other post was about a weekend charter to a popular NC coastal attraction, the U-352, a WWII submarine sitting at 115 feet, which usually has good viz, or at least viz above 20 feet, moderate current at worst. This past weekend, the viz was apparently terrible, described as less than 5 feet, the water was described as 'black' and equivalent to making a night dive in the middle of the day. I have never seen those conditions on that site, which goes to show how conditions can vary. A usually benign site becomes something else altogether.
The incident decribed in the OP is something every competent instructor worries about, every time they go into the water with a student, even if they don't admit it. I don't care if there are 2 students, 4 students, 8 students, it doens't matter if you have dove the site 100 times before. Every dive with students, for whom you are responsible - certified divers doing an advanced course, or brand new OW students, is a challenge and it doesn't matter if you have a CA (DM, et) or not.
I was completing Open Water certification dives with a private student several weeks ago, in a local quarry where the tempratures have dropped from the summertime values of 75 degrees, to the mid 50's, and the vis has dropped to 6-8 feet. I have been there dozens of time, with (literally) hundreds of students, so there still should have been some level of comfort, even wioth the temperature and viz. This time, the student and I were swimming side by side (he did a great job of following directions about staying close), I was checking him every 20 seconds or so, but at one point I turned and he wasn't there! We were only at 25 feet, and I thought he may had lost buoyancy control and ascended. I did a quick 360 in the horizontal plane - no student. I looked up and to my left - no student - then up and to my right - no student - and I felt like my heart rate went from 70 to 140 in a flash. Fortunately, he was right above me, had tried to move to my left when he saw me turn, then tried to move back to my right when I turned my head back, but was behind my head movement both times. In the end all was well. But, it scared the dickens out of me. In 30 seconds, the situation changed from a comfortable (cold, but stable) dive to 'OMG, where is he???'
So, I agree with the calls for patience, and avoidance of speculation. I can naturally appreciate questions about where the diver's buddy was in all this. But, we have little information at this point about how the incident evolved. In addition to genuine feelings for the victim's family, I have concern for her dive buddy who could not help but be affected, and I also have empathy for the instructor, whomever it might have been. Hopefully, we will learn more about what happened.