When I'm climbing I feel no hesitation in talking to people who are not safe. But in that case, there is a direct cause and effect between getting it wrong and serious injury. There are some people I just won't climb with and I've had a proper go at people letting their kids run about at a climbing wall like it's a soft play.
But in this case, no direct correlation between actions and danger and as said above, criticising a man and his petrol toy is a dangerous occupation. Maybe a cheery "that looked like a struggle, what was going wrong" and take it from there depending on response.
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