This is a sad story all around. Obviously the woman should never have been left alone, but that being said, perhaps the husband was either not as aware as he should have been, or in denial as to the gravity of her dementia.
This however does not excuse their treatment by the cruise line. Husband should have been allowed back onboard to discuss the matter and details of the plan. My heart goes out to this poor confused woman who would have been even further traumatized by being so quickly and unexpectedly removed from the ship. Could have been handled better IMHO.