I'd send them a letter and tell them that:
1) This debt is out of statute (I'm sure it is after 10 years but you'd better double-check)
2) And more importantly, he is deceased, his estate has been probated, and they did not file a claim against the estate in a timely manner.
If your name was on the account it shouldn't hurt you. If they come back again demanding money, let us know and we'll figure out the next step.