At this point he had two options, lose his temper and go thermonuclear on the driver, shouting and swearing at him for his dangerous incompetence risking escalating the situation to even more dangerous levels, or acknowledging to the driver that he was OK and wave him on his way.
When I heard my friends story I had to congratulate him for his self control, and let him know what a great boost he had given the image of cyclists in South Australia.
By responding with a friendly gesture the motorist suddenly saw my friend as a living breathing human being, not "one of those" Lycra clad, insane, foaming at the mouth, feral cyclists. My friend had made the situation personal and the motorist could no longer view this as another random encounter with a self important, hipster, road hogging cyclist.
Had my friend turned abusive, or angry and escalated the situation all the motorist would remember after arriving at work would be a frightening and dangerous encounter with one of those crazy, suicidal cyclists with no respect for the rules of the, and that's how he'd share the story with his co-workers, gaining support from them all. The image of the frightening, rabid cyclist would completely overshadow all other details of the accident.
But by staying level headed and making it personal, my friend ensures that when the driver recalls the situation they remember it as the time they almost knocked that friendly chap who smiled, waved and wished them a good day, off his bicycle. They remember the details that led to the accident, and that they were at fault, and they remember the person rather than the aftermath and the faceless inhuman monster that thumped the trunk of his car with his fist while shouting abuse.
When cyclists get angry and abusive at drivers the situation is no longer about the cause of the accident, to the motorist it is about escaping a frightening encounter with "one of those" dangerous, unhinged, maniac cyclists. Its all the motorist remembers and cyclists are further dehumanised.
Now you may disagree, but I applaud my friends conduct in this case. With one simple gesture he had humanised all cyclists in the eyes of the driver, and possibly removed years of hatred and bias the motorist had towards cyclists in general. Next time that motorist sees a cyclist he sees a person and one thing we need on our roads is more motorists who can identify with cyclists.
I encounter dangerous acts by motorists (and fellow cyclists) on a daily basis, dangerous, but not all of them life threatening. If I were to get angry over each and every one of them I'd have a pretty stressful ride, and become an insanely bitter person. Today I tend to practise a more peaceful, turn the other cheek, style of cycling, Gandhi-like if you will for it was Gandhi who said "Anger and intolerance are the enemies of correct understanding".
I'm not saying that cyclists don't have the right to get angry and stand up for their rights on the road, but not all battles are worth fighting. Pick your battles carefully and we will win the war.