A 401(k) participant names his spouse as beneficiary of his 401(k) plan, then divorces. The MSA contains mutual disclaimers of each party's retirement benefits and is notarized. Its a community property state.

He changes his life insurance beni designation from the ex-wife, to his mom, but forgets to make a corresponding change under the 401(k) plan.

The participant dies. The ex-wife is now a non-spouse, named beneficiary. The mom wants the retirement benefit. Can the ex-wife disclaim interest in the benefit, so the mom can get it? The plan definition of beneficiary says that in the absence of a designation, it goes first to a spouse, then to children, then to the participant's estate. He has no children.

Yes the ex spouse can disclaim the benefits under IRC 2518 within 9 months after death but cannot direct who will receive the benefits. If the ex-spouse disclaims, the benefits will be paid to the estate of the decedent. You will have to consult an attorney to determine if the deceased's mother will inherit his estate if there was no will (or if he left a will whether his mother is the beneficiary).