Assuming an ERISA-governed plan, this seems an illustration of another situation in which the plan’s administrator is not responsible for the participant’s failure (but might, practically, be burdened by it).
David Rigby and CuseFan suggest one recognize that, even after the divorce and after the participant’s death, a court might issue a domestic-relations order. See 29 C.F.R. § 2530.206(c)(2) https://www.ecfr.gov/current/title-29/subtitle-B/chapter-XXV/subchapter-D/part-2530/subpart-C/section-2530.206.
In responding to claims (if any), a plan’s administrator should be punctilious in following ERISA § 503’s and the plan’s claims procedures.