A Chinook is hardly noise stealthy, why didn't she dismount when she heard it coming over the horizon?
Firstly, depending on the topography/wind direction it's very possible that the rider did not hear the helo until it was almost immediately on top of her.
Secondly, it's generally reckoned that the rider has most control over the horse when mounted - and dismounting on a road could very possibly have caused substantially more mayhem. given cars/lorries etc.
Finally, it's her pride and joy, so she's unlikely to abandon it because of what might (or might not) happen.
amid speculation that her horse might have been spooked by military aircraft
It could just have easily been spooked by a plastic bag or a pheasant flying out of a hedge - doesn't sell papers though...
I do speak from some experience, having owned/ridden horses for most of the last 40 years, and been a mil pilot.