How do I ask someone to step down from my wedding party?

My wedding is in just over two months now, and I had asked someone who I thought I could count on to be in my wedding party. While she has helped out enormously in all things wedding, she has turned out to be a terrible friend and room mate, is now moving out of state and never bothered to tell me herself. (There have been a lot of non-wedding related issues happening lately) I don't know if I will ever hear from this person again once she moves. Is it OK to ask her to step down, or should I wait it out to see how things go when she leaves? If I ask her to step down, what is a good way to go about it? I don't want to damage this relationship any more than it already is.