The same thing that she wears to other formal occasions, with different accessories.
The classic black dress.
I wear this dress to weddings, funerals, any occasion that requires I be more dressed up that my usual day to day. Since I only have a couple events like this per year, nobody notices that I am wearing the same thing. I am particularly fond of this black dress due to its classic and flattering cut, it is washable and it has pockets! I love the pockets. I put my Gala ticket in one and my keys in the other. The sandals are mine. The necklace is borrowed. The hairstyle and earrings are what I wear every day.