In what circumstances will a substituted beneficiary of a disability benefit be able to assert that the substituted beneficiary was mentally incompetent at the time of the substitution?

In contrast to the decisions which have denied the former beneficiary the right to assert the fraud or undue influence of the substituted beneficiary, in Waters v. Conselho Supremo, etc. (1918) 38 Cal.App. 360, 176 P. 368, the court upheld a judgment for the former beneficiary in an action in which she sought cancellation of a certificate naming a new beneficiary on the ground, which of several asserted was approved by the appellate court, that the insured was mentally incompetent at the time he purported to effect the change.