NEVER make a scene about someone wearing white to a wedding, especially a wedding dress. That's exactly what they wore it to get. Quietly shaming them while not letting them ruin the wedding is the best route.
Not even shaming her but have everybody avoid her , not say a single word to her , turn your back on her if she tries to talk to you , just make her invisible ....feasible or not depending on the number of guests to get on hoard .
