Everyone needs to know so they can be prepared. It is quite shocking to be faced with it unknowingly on the day.

So many people recommend red wine. It is fun to fantasize but really it causes drama and a scene and is not the best way to deal, in my opinion.

Best options overall to me, uninvite and make sure there is security. They want the attention and a scene deny them the attention.

If the person can't be uninvited ignore them as much as possible at the ceremony. Group shunning is called for. Again, they want attention. Don't give it to them.

Have fun shaming them after the fact. Make sure the photographer knows to take a few shaming and unflattering pictures in the white dress to be posted front in center with the initial shared wedding photos.

For the photos you want to keep that she happens to be in, photoshop the white into an unflattering color

Make an announcement towards the end when the drama doesn't matter, asking people to vote(with cash or via qr code link) into the donation box with the color they would like "the dress, you know the one!" To be changed to. Proceeds pay for the extra Photoshop cost.