I agree with this and tend to think due diligence needs to not only account for the regular course of business, but also for the exceptional circumstance. You'll never be able to accuse someone of not thinking of the exceptional UPSIDE circumstance, of course. The problem is the complete ignorance of the exceptional downside. That said, your parent is right that you can't really do due diligence on "war in Iran." Instead you something like "ok, if there's a shock to the system and 20% of our loans default what does that mean for our business?"