I don't understand why that thread was started on a m/c forum! Far better to ask on a horse forum. If the horse/rider has just p/l cover, it sounds as if it is likely to be through BHS, so even better to ask their advice. A lot of people have insurance cover that they don't think about, through their house insurance or union membership, or membership of any one of a number of organisations.
if a motorbike rider went into the back of my car i would expect my insurance to fight to get the motorcyclist insurance to pay. surely its the same for horse and rider? if not it bloomin well should be!!!!!!
It sounds like horse and rider are insured separately as the rider doesn't own the horse. Owner of the horse should claim through her insurance for the horse, and rider on her insurance for personal injury.