I would never wear white as a wedding guest, unless specifically asked and agreed with the bride beforehand it's inexcusable and terrible manners.
I was at a wedding once where the bride, who is a dear friend but not model-looking, was wearing a pale pink lacy dress that really suited her but wasn't a classic bridal look.
Then some plus one of a cousin of the groom, a girl they'd never met before, rocked up in a snow white long strappy sundress. She looked much more like the stereotypical bride, PLUS she happened to be willowy and stunning, and it was very awkward.
My hunch is that the guest was being a cow, trying to make some point to her boyfriend about how good she'd look if her proposed? Or something. Whatever she was trying to achieve, she definitely affected how much my lovely friend enjoyed her wedding day.
It's a very simple rule to follow and anyone who doesn't follow it, honestly, isn't a very nice person.