Seller might be equally disturbed/upset to learn about the rotation/blocking. What if the pony did founder in Heritage's care and therefore didn't sell when the buyer was told? It would get back to the seller why the sale fell throught... Then the seller might be after Heritage for failure to properly care for the pony.
If the pony foundered under Heritage's watch, and the choices are enjoy a big commission and hide the founder or buck up, be honest, lose the sale, and maybe get sued by the seller.... is it so shocking which option might win out? Heritage was in a better position to keep the buyer in mushroom status than the seller... AND get a commission on top.
Why not? Unless moral obligations bother you.