This is not just one story, but an observation based on over 20 years of boarding experience, almost 10 years as an instructor, and 40 years of horse ownership.
Yes in my childhood we did not have helmets for sports, except maybe football, but it just makes sense when the odds of an animal 10 times larger can jump, kick or spook in the blink of an eye to wear a helmet while ground handling as well as riding. Especially when the handler is a beginner!
I have seen more accidents in our isleway than any in the arena. At my facility if you are under 18, you better have a helmet on within 10ft. of a horse! I always find it interesting that people who grew up riding english never question the helmet rules, while the opposite is true for western.
I grew up riding bareback, but after seeing the every time every ride video years ago, I am a firm believer in helmets.