How can I best help this horse?

tell the owners that you will work with him. show them that he behaves well when you ride him. offer to work with him until he is suitable for other rides. even offer to teach the other riders how to handle him so he listens. tell the owners that if they don't see an improvement in a month or so, then they can do whatever they want with him, but at least give him a chance. a lot of horses get sold to slaughter houses when they're in auctions