Legal Question in Landlord & Tenant Law in California

The landlord is obliged to find an acceptable tenant to fulfill the remainder of the rental agreement's term. She would be responsible for the remaining time left on the lease minus the rent received (or to be received) by a new tenant, plus costs of advertising and cleanup.

Whether the landlord will actually sue your friend is unknown, of course. It might not be worth the landlord's while to do so. The lawsuit could drag on for several months if it's ever filed, since possession of the apartment is no longer in issue, so the law does not give the case a priority and speed it up.