In keeping the horse standing, why don't you give the man hanging the reins, so he can hold the horse under him, but maybe after a few days the horses struggle to leave will get stronger and he will finally rip the reins out of the hands of the weak hanging man?
Wait there's hope!
Option 1: Give the man the reins. If the man held the reins he could force the horse to not leave?
Option 2: Tie the horse to the tree. How long would the horse stay there if he was tied? Wouldn't he try to run, rear up, etc? How long before he did that?