I’m a research psychologist who, for 30+ years, has studied the many ways in which the fear of death affects people, as part of the terror management theory which I developed with my colleagues. This research has shown that reminders of death have diverse negative effects on many aspects of behavior and emotion. But we have not directly studied the fear of impending death — this would be impossible and probably unethical to study in a way that would be directly relevant to the situation you describe. But it is widely accepted among psychologists that the threat of impending death is traumatic, and at one time was a necessary component of a diagnosis of PTSD. Some people have done research on how people with terminal diagnoses cope, and as you would expect, most are terrified but some are able to transcend these fears and cope well with their impending death. But this too is different from the situation in your case — a sudden unexpected experience of impending death. So though there is probably no research directly addressing the situation of your client, it is reasonable to extrapolate from what has been studied that this is one of the most terrifying situations imaginable.