It is an unlawful employment practice `[f]or an employer . . . because of . . . sex . . . to harass an employee . . . .' [Citation.] The FEHA also provides that `[sexual] [h]arassment of an employee . . . by an employee, other than an agent or supervisor, shall be unlawful if the entity, or its agents or supervisors, knows or should have known of this conduct and fails to take immediate and approp