OMG I would be furious! What is she thinking?

Even if she was not aware of the custom here, it's kind of common sense that you don't wear the same color as the bride. I don't know of any cultures where anyone else would wear white, even in Asian cultures where the bride doesn't wear white, no one else wears it either, so I don't really think it's a cultural thing.
I find it so hard to believe that the staff in whatever shop she went to didn't say something about her choosing a white dress as the MOG.
I wouldn't even want her to be wearing a color that is too close to white, like a light silver, but that's just me, I want to be the only one in white, even my FG is not wearing white. It's your day and you should be the one to stand out. While no one will mistake her for you, it still will look funny in pictures.
Maybe someone can hint to her that no one but the bride is supposed to wear white here, and kind of tell her in a "looking out for her, don't want her to feel uncomfortable or have people whispering about her behind her back kind of way"? I don't know if you DF would say something to her, but you could go that route so it's not about you being upset, it's about how people will perceive her and she might be more willing to change her outfit. Because a lot of your guests will be talking about her if she wears white and not in a good way
I hope it all works out.