The debt this person owed your mother becomes a debt of her estate. As the executor, you could sue that person--technically, the estate sues, but you are directing the suit and bringing it for the estate. The suit would be for breach of contract, or violation of the terms of the promissory note. The one kicker is, you *have* to hire a lawyer to bring the suit, since a non-lawyer cannot represent another entity--even the estate over which he or she is executor--in court. So if the amount in question is not large enough to justify the cost of an attorney, you may wish to let it go.