At this point its probably better to try and work through it on your own, at least for a while. Emotions run high during wedding planning. Theres a chance that it will blow up and it would suck to lose a friend over this. Have you already booked flights? Will you still be going for a visit in November?
Theres still lots of wedding stuff that will need to be done and I doubt she did this to hurt you. Its just her mum was there and it worked out that way. Mums rank higher than friends in these sorts of things.
If you think shes a chill kind of person you could let her know once your feelings arent quite so raw. But it depends on what you are hoping to get out of telling her, do you just want an apology? She may have already said sorry, will another make you feel better? Do you kind of want her to take the dress back and redo it in November? Probably not going to happen, that horse has bolted. Just to vent to someone? Then shes not the person for that right now.
Ultimately you know her and your relationship with her better than an internet stranger. But telling her while your emotions are raw and shes in the middle of wedding planning with frazzled emotions of her own seems like a good mix for a big argument.
Yeah, I want to be a part of the solution but I never feel like I have anything of value to add to the discussion.