I believe this guest is posing as disabled and that the dog is not a service dog. A service dog is not going to be trained to find truffles. While the dog is truffle hunting the guest is over in the bushes seizuring unattended? Really?
Some states are making it a misdemeanor with a big fine to pose as disabled and to pose as needing a service animal.
You can find out if the story is true if you choose to do so. Is the guest the attorney or the medical student? You can find out from the court/law firm or with the medical school whether the dog accompanies the person at all times. You would have to be sneaky to do that and may not want to do so but you could. You could also look on social media and see if the person who is not disabled is shown with the dog while the other person is doing something else.
People who pose as disabled and misrepresent their dog as a service dog are scum.
If you allow the booking, you can require that the dog be leashed and within 6 feet of the disabled person at all times. Not out in the back yard without the person, not walked by the other person for more than a couple of minutes and never for one second left unattended in your premises.