I'm with NNF here, line someone else up as the chances of her honouring the booking now are probably slim to anorexic.
If she does get back in touch beforehand, then you've got a choice on who to see - if not, you're already sorted and the Neg on her can follow.
I was going to suggest something along those lines...
The OP will be able to gauge it better than me as I can only go by what I see on this thread... He will know when booking is etc etc... If it were me I'd possibly give it to evening before booking and if still no response I would look to book someone else...
I'm not advocating double booking, not by a long shot, but if by some minor miracle she did reappear - night before, day of booking, whatever, then the OP would just have to tell her he had booked someone else, as her comms were so poor he felt the booking had collapsed, therefore he would have to cancel
their booking...
Where I differ is, I don't see it as a choice... I wouldn't cancel the back up booking as I think that's shit form...