Unfortunately I don't know that there is much you can do. It happens all to often. People who sell a horse because it is slightly to testy for them but then end up buying another horse of equal life experience thinking that things will turn out better than the last time. As long as humans are around, you will get those irresponsible horse owners.
You could try sitting down with them and making a list of all the things that involve horse ownership and all the problems they might encounter with a trained seasoned horse vs a rescue. That might work? There is only so much you can do.