They are out of luck - in the long run. If the account was in his name only then you are not responsible. That is not the entire story, however. For example, if he transfered a bunch of assets to you before he passed and then left his creditors nothing then the creditors would have a case to go after those assets. Since you had a POA for him it is possible that you did this yourself. So in some sense Lowes is right to go after you.
How do you protect yourself? Most states have a small estate probate procedure. In his case he might have an insolvent estate which would be a different procedure. You might be able to open such a proceeding even though you have passed the stated time. It sounds to me like Lowes has not received the proper notices about the insolvency of the estate:
"Interested parties entitled to notice shall include the creditors of a decedent's estate if the estate is
expected to become insolvent and shall also include the State of Tennessee if some
or all of the estate may escheat or be payable to the State of Tennessee. Service of
process may also be required as provided by law."