Legal Question in Real Estate Law in California

Re: Terminating a Non-Contractual housing agreement

Both would work. She can serve a 30 day notice that she is terminating his tenancy, and she could sign a formal rental agreement with you. If he persists in returning, a restraining order can be requested.

Re: Terminating a Non-Contractual housing agreement

My general sense of the matter is that an eviction process is more appropriate as a starting point. By the way, it is not necessarily a "non-contractual" arrangement. In addition to written contracts, the law also recognizes and often enforces oral contracts and contracts implied from the facts and circumstances even when nothing is written or spoken about the deal. Finally, courts also recognize and enforce so-called 'quasi-contracts' which are not contracts at all, neither written oral nor implied from facts, but based upon the legal principle that one should not be 'unjustly enriched' at the expense of another who has provided something of value from which another benefitted and nonpayment would be unjust.

You might need a restraining order to back up the eviction, but let's hope not.