Does she have any legal recourse at this point to receive his retirement in the event of his death ?
Your father should contact the human resources department of his former employer and see if it's possible for him to designate her as his beneficiary now.
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Ms. Koslyn gave you good advice. Also ask whether your father's plan automatically designates your mother for survivor benefits.
These comments do not constitute legal advice. They are general comments on the circumstances presented, and may not be applicable to your situation. For legal advice on which you may rely consult your own lawyer.