I have a client that has a retired participant who refuses to cash her pension checks. She thinks someone will steal her money and doesn't trust anyone, including her family members. She has no bank accounts so direct deposit is not an option.

Her family has not been successful in establishing a power of attorney.
However, we were informed that her son was recently assigned Representative
Payee by Social Security.

I ran this past several attorneys but none of them were familier with the term "Representative Payee" or their authority.

Can the Representative Payee cash the pension checks? What would happen to the uncashed checks if/when she dies? Has the Plan met it's obligation? Could/should they give her cash?

A plan cannot force a participant to cash a check and after a period of no more than 6 months it will be uncashable under banking laws. After the partciipant dies the representative of the participant's estate can apply to the plan to for the lump sum value of the payments that were not cashed. By the way the participant is taxed on the amount of payments paid in each year even though they are not cashed.

In California, there is a court procedure to appoint what is known as an elizor who then signs legal documents for the party. This is used in litigation, for example, when a party to a lawsuit refuses to sign required forms or documents. Not sure if this will work for your situation but it might be worth investigating further.