Wow. Glad he's OK and will live with only a scar.
We all know but sometimes need these reminders that accidents happen. I by no means live the most adventurous outdoor life but like all of you, have had my moments. I have been lucky, more often than not, for two reasons. 1) I try not to be outright stupid or careless. 2) I routinely remind myself with a little self-mantra of avoiding those "low-probability / high severity" scenarios... such as firearm mishaps, free-falls, heavy or falling objects, lightning, cold or fast water, and other similar hazards with no escape route/backup plan and little chance for a good outcome. If my action could result in fatality or severe injury and involve relying on something somewhat out of my control (a slippery handhold, rushing water, loose rock, old or overloaded items under severe tension, etc) I'd choose to find an alternative if I can or if not, double down on my concentration and care. This leaves me of course always with the chance of a true accident, but it helps to consider the "what-ifs" in this way, while not having to be afraid of every little thing. So have fun but be safe everyone!