This may be coming a little late but I wanted to add what I would do. First I would call everyone that I already have booked and let them know NOT to cancel anything without speaking to me face to face. Then I would call the "snot" back and give her the name and number to a photographer. The absolute worst one that I could find!!! AND. I would gush about how much I LOVED said photographer. :) Sorry, I would play dirty in the nicest way possible.
I, believe it or not, can believe that someone did this. It is a major reason why the only people at work, who know my exact date, are those who will be standing up with me. I don't necessarily like secrecy, but I also do not believe it is right, or like getting screwed with.
I truly hope everything works out well for you (OP) and your FH.
I suggest that next time she asks you for vendor referral ask her why. Remind her that she set her date for the same date as urs and DJs and photographers can't do two events in the same day. Seems like she has been playing dumb so you should call her out on it. Then I would go on to tell her that it really hurt me that she would pick that date after knowing that you already picked it and then proceeded to tell you that you had to change your date. Tell her that this is not good friend behavior. If she makes a snarky comment or still refuses to change her date I would cut her out of my life. With friends like her who need enemies? Yes she's your FH's boss's future wife but that doesn't mean you have to socialize with her...just polite at company events.