You never know what to expect in one of these threads. You certainly have gotten a wide range of viewpoints here but in thinking about the reality of rigging a preventer on a charter boat it would seem pretty unlikely to find the resources aboard to rig one in a manner that would not endanger the boat.
Properly rigged a preventer needs to be quick and easy to release from the cockpit and the line should be long enough to permit the boom to cross the boat to the other jibe. It should be a nylon rather than a dacron line, and should ideally be attached near the near the end of the boom and lead to a block near the bow. Generally that means a line that is twice the length of the boat rigged (or at least capable of being rigged) on either side of the boat.
Having been aboard boats where the boat jibed and broached, leaving the boom held by the preventer abeam on the new windward side of the boat, or where we dipped a boom held by a preventer and tore the gooseneck apart, perventers need to be stretchy and quick to release. Anything else, endangers the vessel and the crew far more than not rigging a preventer.
The reality is that the line and the blocks to properly rig a preventer simply will not be aboard most charter boats. So what should you do if you can''t rig a preventer.
-Don''t head dead downwind if you don''t have to. Reaching is generally faster,offers a better VMG and a lesser chance of a jibe.
-At deep reaching and running angles, pay attention to the boom and mainsheet and always move around or sit in a matter that assumes that the boat could jibe at anytime. As you walk up the deck, keep an eye on the boom, be prepared to duck, as you get near the boom place your hand on it as a way to guage your position relative to the boom and its likeliness of moving towards you.
-As helmsman, you need to really concentrate and while accidental jibes will happen to the best sailor, staying alert at least means that the helmsman can shout a warning. As helmsmen you need to take a mental inventory of where everyone on the boat is located. You are usually in the best position to see who is at risk and it is not inappropriate to say, "Peter, we are on the edge of a jibe so please stay low as you move about the boat and keep an eye on the boom." (I wish I have had a penny for everytime that I have said something like that in my sailing career. I could buy a newer boat for that kind of cash.)
I would not assign fault here. There are things that you both could have done to minimize the likelihood that you might have gotten hit by the boom.