Legal Question in Real Estate Law in California

That sounds like a very bad idea. They could not live together as man and wife but want to do so as roommates? Is the value of the rental going to be taken into account as to alimony? If either is going to be seeing others romantically how do they explain the situation to their dates?

If the divorce is final, then she has no rights to the property. Community property exists only for married couples.

An ex-wife is a stranger in the eyes of the law. She should pay rent and sign a lease or rental agreement.

Does your son have any obligation to her for alimony, child support, or payments under a marital property-settlement agreement or order? That might affect whether she should pay rent or not and how rent paid would be treated.

There are cases in California where girlfriends have claimed, and been granted, rights in the boyfriends' property -- see, for example, Marvin v. Marvin -- but these cases have involved express or implied promises, and other special considerations, going beyond the living-together arrnagement (if that's what would be happening).

In any event, your son seems to be in a position to call the shots, and he shoud get a written agreement and preferably one where she doesn't get charity.