I understand not wanting to back out. I have a similar friend and though sometimes I wonder why I stick around, there are times that we get along great and have an awesome time together. My friend isn't as demanding as your friend, though (she likes that I'm 45+ lbs bigger than her), but she has hurt me deeply before.
My solution was to "contain" the friendship. I didn't share certain stuff with her, I didn't expect certain stuff from her (for any real support), and I won't give her certain things (my trust). It works out fine. I'm not disappointed, and she isn't expected to give more than she can.
I say that if you ordered the dress in the size that will actually fit you at the wedding even if you lose no weight (or thereabouts), then the bride doesn't have to know if you do or do not lose weight. Don't talk about it. If she demands to know your progress or that you go to the gym, say something vague and change the subject subtly. It works wonders for me.
But yeah, don't do this for her. And I understand her demands being a disincentive. It's funny how orders to lose weight = no motivation, but when someone says the equivalent of "you don't have to lose weight" = motivation.
Good luck with your friend and her wedding, and good luck with your own weight loss. Congrats on your loss so far!
