Things like this are very much dependent on state law.

First, though, I assume you are not talking about assuming the debt yourself, you are acting as executor of the estate, yes?

I know in California the debtor must make a claim within 1 year of death. However, in addition, the estate is required to notify all known creditors of the death within a certain amount of time, did you do that? Since obviously the estate should have known about this creditor. (But this is California, I don't know about other states).

Most states also require an estate notice to be published in a newspaper.