I don't think you need to panic - thousands of horses get ridden on the roads daily with no problems.
Like everything, it's only the rare times there's an accident it's on the news.
No one gets on the news if they've been out for the perfect hack.
But I don't think it right to stop people riding on the roads. I feel it is a part of a young horses education.
If riding is limited to those with arenas or who live next to a bridleway you are really only allowing rich people to ride at all.
There's no law against selling, the horse may have a deformity and not be in pain, or there could be any number of innocent explanations. But you cannot force someone to keep a horse. All anyone can do is hope a sympathetic person buys him. Yes I have seen the ad, BTW. A bay gelding.
Why are you worrying? Surely what she does is up to her, she is in her 20's.
I was probably exploited as a teenage working pupil. Would I have wanted someone sticking their nose in? Certainly not!
I have bought several super horses over the years from Donedeal. It's like everywhere, sales included. Scroll past the unsuitable ones, a horse to suit will come up eventually.