Dress to balance the tux, in full cocktail. If in doubt I wouldn't do a ball gown, but a fancy cocktail should pass even if the other women are in ball gowns. If he can find out if the women are in ball gowns that would help. He also needs to find out for you if the women wear hats. The bigger social sin would be to show up sans hat than to show up a little under-dressed.

Good news! No hat needed!! Although depending on the weather, I may be wearing rain boots and carrying my heels.