Legal Question in Real Estate Law in California

I don't believe he could make a successful claim. If you were married, there would now be some community-property interest in the vacation house. If he had contributed to the down-payment, he might have a partial equitable interest under a purchase-money resulting trust. Under the circumstances you describe, the deal was payment of rent, and that's it.

On the theory of 'anybody can sue anybody for anything', he can claim an interest, right or wrong, win or lose, and you get to fight it out in court, or settle it. This is probably a loser for him, but that doesn't stop some people. Have an attorney write him a strong letter, to try to dissuade him from trying.