Aresearcher conducts a focus group to learn about attitudes towards hygiene and disease prevention. two weeks after the focus group, the researcher learns one of the subjects had a heart attack at home and was hospitalized, but made a full recovery. based on dhs regulations, the researcher report this event to the irb?

I believe the answer is: No, this does not need to be reported since it is unrelated to participation in the study.

Unless a person is induced with chemical substance, The cause of heart attack is most likely diet and lifestyle that has been building up over time.

Since the research is revolved around attitudes towards hygiene and disease prevention, they would only observe the subject's behaviour, not making them digesting any substance.