Hello Felicity
That's rather arrogant of your guest - terms of BnB are quite clear. Personally I wouldnt' accept the change.
I would send her a message and say you appreciate she has had a last minute change in her arrangements, but unfortunately you can't accept her proposed change as you are unlikely to be able to get another booking this late in the day. If you like offer her a discount for another stay - valid during an off peak period when you are unlikely to be busy.
Hosts shouldn't be held to ransom by guests leaving bad reviews. If she does mention it, then you can explain why you weren't able to accept the change.