I've been seeing this more and more on Airbnb - guests not wanting reviews. It strikes me as odd and underhanded.
More than once it occurred to me that it was a way for people to have illicit affairs that normally would be discrete in a hotel. Personally, I don't provide my space for people to come and get it on, we provide a space for people to stay when traveling. Certainly Airbnb is a nicer way to go than a cheap hotel, but is this what you want in your home?
You could turn a blind eye, not write her a review as requested, and leave it at that. Maybe write a little note to her that you don't want her to come back, if that's the case. Personally, I would tell her I was uncomfortable not leaving a review so that other hosts can be aware of the type of guest she might be for them in their home. My sense of obligation to provide information to the host community would outweigh my sense of keeping her tryst private.