Harbonah knew this either by his own observation, or by the information of some of his brethren, who were lately sent to Haman's house, Esther 6:14, where they might easily see it, or at least hear of it. And this he said, either out of a distaste which he had taken against Haman for his great insolency and barbarous cruelty; or in compliance with the king's inclinations, and the queen's desires. Had spoken good for the king, even to the saving of the king's life, Esther 2:21-23, and therefore deserved a better requital than this even from Haman, if he had not basely preferred the satisfaction of his own revenge before the preservation of the king's life.
Related Commentaries of Esther 7:9
Esther 7:9
9 And Harbonah, one of the chamberlains, said before the king, Behold also, the gallowsd fifty cubits high, which Haman had made for Mordecai, who had spoken good for the king, standeth in the house of Haman. Then the king said, Hang him thereon.