Alot of adverts state that EI members and regulars will take priority.

I guess that can only really mean take priority over someone else already booked, and if she's perhaps being offered substantially more income as the result of an overnight then I can see why she might opt for that. I guess at least she was honest with you and could've just said she wasn't feeling well or something else.

I know that doesn't make your situation any better - and I don't agree with one side or the other, perhaps there are others out there that would entertain a short notice booking?