Fuji, it might be useful to talk to her in person when nothing else is planned--not at the florist or the caterer or the dressmaker. Try to get her in a reasonable moment. Then just be honest and tell that she's gotten a bit out of whack with this whole thing. Suggest that she write a list of the tasks that she needs company/help with, and a list of the MOH and bridesmaids. Most of the accompanying should go to the MOH. Since you're nice and helpful (but not a doormat), suggest a couple of things that you can help with. In other words, help her make it manageable on paper and then set your own limits.

She sounds really dreadful. She probably doesn't even realize it. IMO, people get obsessed about wedding planning because it's the only thing they can have control over. In some ways, the whole concept of getting married is one of giving up significant control of your life. Sharing your life can be wonderful, but there's a huge scary part to it, too. Maybe there's underlying stuff going on with your friend. Unless you can get her to a shrink, there's not much you can do about that other than listen if she has concerns.

Best of luck to you and your friend.