The pasuk in Megillat Esther 5:14 tells us that וַתֹּ֣אמֶר לוֹ֩ זֶ֨רֶשׁ אִשְׁתּ֜וֹ וְכָל־אֹֽהֲבָ֗יו. The word וַתֹּ֣אמֶר means and she said, so the pasuk would be grammatically correct if only Zeresh was speaking. However, the pasuk says that Zeresh and all of Haman's loved ones were speaking, so the pasuk should have said וַיאמֶרוּ לוֹ֩ זֶ֨רֶשׁ אִשְׁתּ֜וֹ וְכָל־אֹֽהֲבָ֗יו. Why is the word וַתֹּ֣אמֶר used incorrectly?
The word וַתֹּ֣אמֶר applies to זֶ֨רֶשׁ as the one who spoke. It would idiomatically translate
Zeresh his wife told him and all his loved ones chimed in.
Thus, the singular applying to Zeresh shows who the main speaker was and that she was the one giving the advice.