Cal. Ins. Code § 555

If a policy requires, by way of preliminary proof of loss, the certificate or testimony of a person other than the insured or beneficiary, there is sufficient compliance with the requirement if the insured or the beneficiary (a) uses reasonable diligence to procure the certificate or testimony, and (b) in case of refusal to give it to him, furnishes reasonable evidence to the insurer that the refusal was not induced by just grounds of disbelief in the facts necessary to be certified or testified.