It is a hard call, but I think it is better to err on the side of disclosure. There are countless posts on this site and others about how hard it is to get information about scuba accidents. In fact, the operators of this site solicit contributions for defense of a lawsuit brought, as I understand it, by a dive shop unhappy about comments made here about accidents that may have been associated with the shop.
I am enjoying my time with Scuba Mau, and think they are a good op. The facts as described suggest the diver experienced some health problem, e.g., an MI, while underwater and panicked.
Still, I would have liked to have known. On my first dive Monday afternoon, the DM appeared distracted underwater and kept leaving me and the other diver behind to check on a DM-in-training who had another group. At some point, my buddy (who I only met on the boat out) and I exchanged the universal WTF sign, and finned back furiously in the current to get back with the DM. Now, he may have been similarly distracted under normal circumstances, or he may have been overly worried because of the accident that day (if I have the timeline correct). Either way, of course, he used bad judgment. If the DM-in-training needed watching over, the DM should not have split us into two groups in the first place.
Aside from that dive, safety practices have been first rate.
In any event, I would have felt safer diving w. SM if they freely disclosed that an accident had happened that day.