I’ve read about a marathon runner in his 40’s who was put on a ventilator & people with no underlying health problems dying with covid19. But there’s plenty of stories like your elderly friend, centenarians & obesed smokers who only had minor symptoms. Only time will tell what’s the mystery with covid19 but considering HIV/AIDS has been around for 40 years & it hasn’t been eradicated yet we might have a long wait. I wouldn’t be surprised if different strands are more deadlier than others? That would explain why different cities in the same country with similar standards, equipment & practices would have higher & lower mortality rates than others.