Okay, let's see... my thoughts on this one:
I've been privileged in my diving, as every boat I've been on has called roll religiously. (And that's on top of the other layers of verification.) I plan to maintain a purely academic interest in stories of divers being left, at least if I have anything to do with it (and I do).
As for leaving a diver and not even noticing until on a second site, I'd consider that "gross negligence". To me, that would not fall under waivers, as a reasonably competent person would be expected to know whether they had everyone onboard. If a diver was swept down current and found missing *at the first site*, and if that was reported then, I would consider that to be part of the assumed risks of diving. To leave the site without noticing is a different animal.
Still, while there may have been gross negligence, the suit as described is far beyond reasonable. It is laughable to blame skin cancer on one exposure, for example. While I certainly empathize with people who have been through traumatic experiences (having had some doozies myself), at the same time, "Just get over it, already!" is my default response. Perhaps it was traumatic enough to require a solid month of shore diving on Bonaire in order to work through it, and with all those expensive dinners, round it up to $10k, but $4m? Inconceivable.