In my experience, a lot of people who say they will come to your DW don't actually follow through. Everyone thinks it sounds like fun and would love to take a trip when they first head about it, but when it comes time to actually start booking they back out because of finances or time or a hundred other reasons.
So, chances are she won't go anyways and there won't be an issue. I'd wait it out if I were you. Try not to worry about it in the mean time. When it gets closer to the wedding and if it looks like she is still planning on going, then you can talk to your FI and have him suggest to his friend that he bring his GF instead or just ask him not to bring her.