Legal Question in Real Estate Law in California

The law provides a process called "partition" to end unhappy property co-ownership situations. The name for this special kind of lawsuit derives from older times when it was often used to divide up farms and ranches between the heirs by physical subdivision. We are a largely urban population now, plus we have zoning and restrictions on subdivision, so more often than not, partition is done these days by selling the property and "partitioning" (i.e., divvying up) the net proceeds of sale. So, if this property sold for $600,000 net of paying off any mortgages, commission, etc., your mother would pocket $200,000. The owners' shares will also be adjusted by the court, if necessary to be fair, to take into account past inequitable division of ownership costs and/or rental income.

Partition can be time-consuming and costly, but sometimes when one co-owner initiates the process by filing suit, that will result in a buy-out offer from the other, or at least an agreement to sell the property out-of-court. Sometimes the co-owners agree to an out-of-court settlement in which the property is sold and an arbitrator is used to hear arguments and decide how the net proceeds of sale should be shared.

Unless there is something in the marital settlement agreement or decree, or a private contract, I doubt your mother has a legal remedy other than partition. Even if this were a business partnership, which it probably isn't, the majority partner would be allowed, within reason, to set rents, etc. Co-owners of real estate have been held to be in a fiduciary relationship, which could be an alternative pathway to demanding fair treatment, but I'm somewhat skeptical that asserting a breach of fiduciary duty is the best pathway here.

There is always the possibility that Father is fibbing about the rent he's receiving, or maybe the tenant is his girlfriend, in which cases this is probably actionable as fraud, but I guess there is no evidence of either.