What may be relevant is that the low air density due to the high humidity in foggy conditions would reduce both engine power and rate of climb. In the event of an engine failure, A PA30 pilot might have found himself in trouble.
Being the close friend of a relative of the deceased, I am not inclined to speculate on the cause of the accident and have no axe to grind. The pilot and aircraft were competent in the weather conditions prevailing at that time.