Be polite but maintain emotional distance, you may want to point out to her that she is being unfair but it is hardly worth doing so. Don't let her involve you beyond the casual and don't let it upset you.
It could be that the reason she is acting unfairly is that due to your imminent departure she is having to cope with her feelings and finds it difficult but equally she could just be a manipulative sort so I wouldn't want to read anything into it. Without wishing to sound condescending - age may have something to do with it, I rather suspect as you get older your feelings will be more measured.