What does your sister think?
Truthfully, Arabians aren't some other species - they are horses. They are very smart, very sensitive, and often form a bond with a certain person, but the basic training is the same. Your sister ought to have some 'feel' about this horse.... is he actually crazy, or does she think that maybe he's lacking somewhere in his training?

(I have actually known of another horse, who was an unapologetic bucker, and the owners sold her to as a bucking horse to a rodeo string. That may also be an option.)