@Kenneth12, thanks for acting as a backstop.
I'm trying to not suggest he do anything against tenant regulations.
I am assuming the lockbox belongs to the owner.
I told the owner to provide the tenant with a key. Is that illegal?
I am assuming that putting up a sign letting AirBnB guests know that Short Term rentals is against the law is not illegal. I figure the city government would not prosecute the owner for letting guests know what the local law is.
I'm not telling him to confront the guests, but I believe it is entirely appropriate to put up a sign. If the guests want to enter the property under those conditions, they are free to.
I also don't think the owner should threaten anyone.
He should just begin the normal, legal process of eviction. No threats are needed.
I'm sure with both of us helping, the owner can find a sensible and legal path to follow.
Thanks for your advice and suggestions!