Even if your groundman had a helmet on, the branch butt still might have killed him. Short of wrapping yourself in 3-foot thick bubble wrap, there's only SO much protection you can wear before you're too constrained by safety measures to perform the job ably.
Safety measures used in arboriculture such as hardhats, chaps, steel-toed boots, chain brakes, 3-point rule climbing, chipper control bars, and many others are still in reality subjective measures that arborists around the world all employ with varying vigillence. Yes, ANSI Z133 safety standards exist, but they are not observed worldwide, they're not perfect, and due to advances in technology, safety is a continuously evolving concept that is never static.
Fifty years from now, I'm sure the standards for safety will be much different from those we have today.
In an imperfect world, there is no such thing as the perfectly safe working environment. Even in Europe and the UK, where safety regulations are the strictest, fatal accidents happen. It is impossible to be completely eliminate risk; you can only mitigate risk.
Freak accidents happen. No one can ever predict WHEN one will happen, if it will EVER happen. Two different tree companies may adhere to roughly the same safety practices, and one may go 20 years without a serious accident, and the other may have two fatalities in 20 years. The difference between them is simply a quirk of odds. One just happens to be luckier than the other.
Of course, companies that don't pay attention to safety will in the long run have more accidents. But the law of probability can be a funny thing. Very unusual things can happen that one would never expect. That's why there's such a thing as a royal flush in poker.
After reading your story, I'm confident, even though your groundman wasn't wearing a hardhat, what happened was a freak accident -- not an accident waiting to happen. When you had a vocal response and made eye contact of readiness with your groundman, you practiced reasonable care in making sure that limb would be cut safely. That's all you could reasonably do.
There's nobody at fault here.