Ask H&H: riding without a hat

Q: WE run a small DIY livery yard as a sideline to our farming business. A new client arrived recently with two horses, both of whom are sharp competition animals. Our worry is that she does not wear a hat when schooling or hacking out. She is clearly a competent rider — but this is the harvest season. What would our position be if she were to fall as the result of her horse being spooked by machinery, especially if she or the horse was injured as a result?

RB, Wilts