Do not go. Since she is moving away, there will be no repeat interactions, so you are not invested in any way in a potential relationship with this woman and cannot possibly gain anything from attending. Were she a work colleague relocating to another office within the same company and you were obligated to work with her in the future or else risked losing your job, I can see doing something that is unpleasant to you to maintain the decorum and pretend things were ok when they aren't. But you are not obligated in any way and you do not risk losing important assets in your life via not attending. So it is between going and losing your precious time and feeling hurt and not going and losing nothing and gaining an opportunity to do something pleasant for yourself instead, as mentioned above. The choice is clear. And I also agree with the above: do not say that you are not going to attend because this woman has backstabbed you. Just say you already have committed to another engagement on that date and regret you would not be able to attend. If you want advice on how to word it, I am sure there are plenty of good etiquette resources on the web on how to send your regrets when asked to RSVP.