I had an (webcam) interview with booking.com a week ago.
The subject was an improved system on how to offer and charge extra services.
One of the issues was exactly this.
Things like a pet fee, cleaning fee etc. were not shown in the reservation price, but added in small print below the reservation price, the guest will not pay for these services at reservation, but upon arrival.
I told them this is a huge issue. Guests will blame the hosts for luring them in, with a low price and charge them with hidden cost upon arrival, because booking.com is not clear on this.
In you case I would call booking.com and have them contact the guest and explain the situation.
That booking,com is trying to lure guests by showing incomplete prices is not your problem.