I don't know what to say...
This is a tragedy. Condolenses to the family.
I don't really know how to feel about this one. In my logical mind, this seems like an "Act of God" incident. There is danger in anything you do. A badger could have startled the horse just as easily with the same results. (Do you have badgers in the U.K?) How can you definitively say the helicopter was the sole cause for this disturbance? How do you know that a bee sting didn't cause this accident? You can't, but when you are in anguish over the loss of a loved one it is natural to assign blame.
Even if there was no doubt that the helicopter was solely responsible for the disturbance, what does that mean? Just like diving or any other sport, you assume a certain level of responsibility for the associated risk with that sport. Divers with years of experience die each year. I'm not sure why this is different. If you are riding next to a local road, wouldn't you expect that your mount might be distrubed by automobiles? Why is a helicopter different? Perhaps Flying Lawyer can offer some clarity?
(I don't mean to come across as insensitive, but this is a quandry)