(Posting a review in that particular situation might perhaps be not such a brilliant idea.)
But what if the situation comes up? I think experienced punters should perhaps be able to offer more than shutting the stable door after the horse with a big cock has bolted so to speak. Don't punt in your backyard is fine in retrospect.
There is a moment before a punt gets underway and a moment at the end... it is when both punter and prossie are close to that point where they switch back to 'normal' life. It's also the time when both parties size each other up and, afterwards, probably make an almost unconscious decision about the time spent.
At the moment before, there might not be too many options, but seeking out a moment of sincerity is never a bad idea. Take the situation described in the OP. Perhaps you both recognise each other at the same time? What you want is that secret explicitly or consciously shared between the two of you. (As opposed to one person thinking, "I know his/her dirty little secret.")
So you hold the eye contact for a fraction longer than usual. Without negating your lust, you have some sort of attitude of goodwill or caring towards her (this is simply to form a bridge, a bond, as you would in any negotiation). This needs to be strong enough, even fatherly enough, to overcome any shyness or irritation she suddenly feels. Smiling, you maybe say in a low, reassuring voice, "Are you ok with this...?" If she smiles back and assents then you have a starting point for a 'shared secret'. If she says no, then you either need to think quick or skip forward to the 'closing moment of sincere agreement.'
For the closing moment, you want to again reaffirm your secret agreement (and bear in mind that if you haven't played it that well, or even if you have, she may just want out of there, you have to judge it.) You might give her a gentle embrace as she stands ready to go, facing her and looking into her eyes, in a friendship rather than sexy way, a warm but tentative smile hovering on your face and in your eyes, maybe thinking, "this could be worse for her than it would be for me" (or some such shit: you just want her to read the frame of mind that you want her to read!) and say, "I guess this has never happened...", holding her gaze, raising one eyebrow until she agrees.
Every situation would be different and reactions adjusted accordingly, but separating off the punt psychologically is what you would both want (unless she's a devious man-hating cow). If she is a cow, there are numerous ways she could still get at you so be prepared with answers. Of course, if you have "I love Maggie Thatcher" tattooed on your cock (or any other intimate distinguishing feature she might remember, it will be more difficult!"
Why and what could she say. Something like "I heard from a friend that he... " Malicious. In which case you simply point out she is being malicious and either saying something nasty for some unknown reason or is a bit off her rocker and ridiculously mistaken. Kindly outraged, nothing over the top. Women tend to believe other women. To your sister, "A moment of idiocy, don't know what I was thinking, mates put me up to it" etc etc, or combine the two "Mates put me up to it, but when she walked in I came to my senses and sent her away again." (You would be telling the truth according to your secret agreement: the fact that she had reneged is no reason for you to lower yourself!) You could even play the reformer: "I somehow suspected she was doing this and "booked" her to make sure and try to talk her out of it; naturally, nothing happened." Mr Respectable vs the girl who's sunk to Prostitution.
Lots of ways. Think it through. Probably nothing will happen but be prepared. We don't want to read another "ruined my life" post. Too dreary!