Esther 5:10

5:10 But Haman restrained himself and went on to his home.

He then sent for his friends to join him, along with his wife Zeresh.

Esther 5:14

5:14 Haman’s wife Zeresh and all his friends said to him, “Have a gallows seventy-five feet high built, and in the morning tell the king that Mordecai should be hanged on it. Then go with the king to the banquet contented.”

It seemed like a good idea to Haman, so he had the gallows built.