May I gently suggest that if he pays her to have sex with him, then he is not swinging, he is punting. It is just that the punt is taking place in a venue where others are swinging.
I hired an escort to have sex with me at a venue near Heathrow one time. This punt was at an outdoor sort of naturist place. It was a bit awkward since we needed real ID to get in and had to pretend to be a couple. I was comfortable enough with her that I was not worried that she knew my real name and saw my passport. A bit of a risk I know. Was a good enough time that I did it again a month later, but after that, itch scratched.