what happens to my father retirement money after he has passed on?
Since you didn't tell us who the beneficiary is, we can't tell you.
His wishes should have been noted with the plan. Otherwise, he should have made provisions in his will. If his wishes are not spelled out in either document, the residual beneficiaries may be entitled to the retirement benefits.
If you are a potential beneficiary, you should collect the relevant documents noted above and schedule a consultation with an attorney.