I did not know this rule everyone knows and wore a short white linen dress to a wedding when I was in my early twenties. My parents never said anything, perhaps they didn't know the rule! Luckily the bride was in a big traditional dress and so no offense was intended and I guess none taken.
Sometimes the interpretation of what's suitable on here for a wedding is very odd, no black, no white or cream, no mainly white or cream dresses with flowers on even though this is so common in summer it's hard to know not what to wear, no green unlucky and on it goes. In this instance though, knowing it's a big deal now I think it would have been polite of her to ask, but I think now it's done, just move on as it's such a trivial thing to focus on about your wedding day.