California Code, Labor Code - LAB § 3859

(a)(1) Except as provided in paragraph (2), a release or settlement of any claim under this chapter as to either the employee or the employer is not valid without the written consent of both.  Proof of service filed with the court is sufficient in any action or proceeding where such approval is required by law.

(2) The written consent of the employer is not required in a claim that is subject to subdivision (b) of Section 3852.

(b)(1) Notwithstanding anything to the contrary contained in this chapter, an employee may settle and release any claim they may have against a third party without the consent of the employer.  Except as provided in paragraph (2), a settlement or release shall be subject to the employer's right to proceed to recover compensation they have paid in accordance with Section 3852.

(2) In any case governed by subdivision (b) of Section 3852, a settlement or release shall limit the employer's claim for reimbursement to the portion of the settlement allocated to the employer pursuant to subdivision (b) of Section 3852.