I think instead of making any static, say something like, "I would love to help you with all the details going into your special day, but I have a lot of things on my plate and I don't think I can devote the time needed to assist you." If a person can not take that then they don't have functioning brain cells and you really wouldn't want to assist her in planning anyway. Good luck.
I have gotten out of two wedding like that. I attended both weddings and gave really nice gifts $$$$$$$.