Legal Question in Real Estate Law in California

A month-to-month lease, like any lease, is an agreement between one or more tenants and a landlord. When two or more people rent property together and all sign the same rental agreement, they are co-tenants. Each co-tenant has the same rights and responsibilities under the rental agreement, and neither co-tenant may terminate the other's tenancy. On the other hand, either co-tenant may withdraw from the agreement upon proper notice to the co-tenant and the landlord. This is because all parties entered into the rental agreement with the right to terminate it after an indefinite term. Thirty days' prior written notice should be sufficient.

It will then be up to the landlord and the remaining tenant to decide whether to enter into a new rental agreement. (I use the term "rental agreement" to distinguish a month-to-month tenancy arrangement from a lease for a definite term, e.g., one year).

In addition to the rental agreement between the landlord and the co-tenants, there was probably some kind of informal contractual relationship between the co-tenants -- which perhaps will be breached if or when your friend "terminates herself" from the rental agreement with the landlord. While she presumably has a right to pull out of the agreement with the landlord on proper notice, there is some risk in breaching the agreement with her co-tenant. Whether the risk is significant depends entirely on their understanding and, to some extent, how aggressive the co-tenant would be in trying to enforce it. I think in 80% to 90% of all cases, it would be worth the risk to give notice and move out if that's what she wants to do.

However, your friend needs to understand that it'd be her deal with the co-tenant, not the deal with the landlord, that'd govern any dispute.