Can you tell me if it's possible to report a POA after the person has passed away?

Are you the executor of your father's estate??

Are you the Executor? I agree, see a lawyer. He can request an accounting of how she handled Dads finances.

You start by taking your evidence to an attorney, who will be the one to tell you if you prosecute it or not, or even have a winnable case or not. Find one who specializes in elder abuse/fraud. You will be paying for the attorney.