David Troup if it isn’t permitted and not a legal dwelling why doesn’t the city require her to move ASAP for her own safety? She shouldn’t have it both ways, continue to live there while claiming it is illegal.
I would never rent on airBnB, too many horror stories

I’m a California landlord. And while some of our laws are a bit crazy, and the woman (tenant) in this case is definitely a grifter, this whole thing is completely the landlord's fault and it was completely foreseeable.
He rented out an illegal apartment that was built without permits and did not have a certificate of occupancy. That’s a recipe for self-destruction and that’s what he got. If you rent an illegal property to someone litigious or malicious, you might as well sign the deed over to them as they may end up owning the property.
The tenant is a scam artist for sure, but the property owner set himself up to be scammed and is a victim of his own greed. He knew the apartment was not permitted. As someone who follows the rules, I can’t get too upset about seeing bad things happen to people who don’t.
I also think anyone who rents property on sites like AirBnB is asking for trouble, as you basically lose your ability to vet the tenant before they move in. A little research on this woman would have quickly revealed the fact that she had pulled that stunt before.
This whole thing is a demonstration of why you need to obey the law and do good tenant screening.

Just move all her stuff out and change the locks! Private renters can get away with that

Interesting legal conflicts.
Guest house was not an authorized registered domicile. Not up to code. Owner violations.
Then remove as trespass. If the "unit" isn't a legal domicile, then it is trespass, not lease violation.
But hey, it is California

Oh California…..

It is so incredibly messed up here in CA. I teach CAM for our associations and so much about rent collection doesn't apply anymore. Keeping up with the law changes is a full time job in CA.