I would try the Transfer System Ownership option in the app where you would input your own email/password.

Hi. This question does not quite make sense to me. He cannot log into his own Sonos system when in your home.  The system is stored on the speakers. His system is on his speakers in his home.

So are we really just talking about a music service account,:such as Spotify?

And what do you mean by 'one of our Sonos systems' ? How many do you have?

Not a solution but this makes a good case for having a “Guest WiFi” that keeps visitors from messing about with your stuff.

The OP’s post made me think their visitor ’may’ have factory reset one of their Sonos speakers and set it up for use with their own Sonos App/Mobile controller - but that’s just a bit of a guess on my part, until things have been clarified.

If true though, then it’s perhaps just a case of FR-ing the device and adding it back to the original Sonos HH.

Personally speaking, depending on the circumstances, I would maybe consider changing the routers WiFi/Admin passwords too, if the ‘guest’ were given those, or perhaps they were able to obtain them from a label on router during their visit.