A few things occur to me:
1. Of course, any action would have to be taken in TN and I don’t think anyone here is competent to comment on TN law.
2. The power of attorney, which would have been subject to LA law at the time, apparently was accepted as valid by the bank. As such, the grand nephew had full legal authority to remove the money. The question would be whether the money removed was used for your father’s benefit. Someone with a POA has a fiduciary duty to act in the best interests of the person giving him the POA.
3. You could inquire about these things through the subpoena power of the court in Tennessee if you wish to contest any of the actions that took place. Again, time limits and types of actions that might be available to you would be controlled by TN law. In LA, for the benefit of anyone reading this thread, you can have money brought back into the estate if you can prove misuse or breach of the fiduciary duty.