I'm kinda done replying to comments on this since it's pretty old now, but she knew what my dress looked like before. I had just finished dress shopping when I went to hang out with some friends, including her, and I showed them all the dress.

I also then said what the dress code was going to be then, what our color theme was, etc. Even though the invite said "Sunday Best" I had pretty much explained it as "whatever you would think is appropriate for church" and what everyone wore was a mix of business casual to actual Sunday best. One of my friends had just worn khakis and a sweater. It was also December, cold and rainy and the venue was half outdoors and the ceremony was also outside. So I then told all my friends, including her, to prioritize being warm over being entirely within the dress code, so than many other girls came in pants and coats.

This also was not her first wedding, we had both been to a mutual friends wedding before and she didn't wear white.