In the previous circumstance in which Haman asked his family and friends for advice (Esther 5:10), he had to summon them. According to Ibn Ezra, Haman did not need to call for them now because, after having recommended hanging Mordechai and going to Achashverosh, they stayed around to see the results of their advice.
The M’nos HaLevi suggests that they had left, but returned to comfort Haman upon death of his daughter and the other terrible events of the day. He also writes that Haman only told Zeresh, but word spread.
In the simple explanation, this information comforts Achashverosh because he sees that Mordechai is on his side.
Furthermore, as the Alshich points out, Mordechai is mentioned directly, without mention of Esther. According to Imrei Shefer, this alleviated Achashverosh’s paranoid concern about a Mordechai-Esther conspiracy to kill him.