Got a notice for a personal loan taken by my father who expired

My father was expired unexpectedly six months ago and my mother took over his HP retail outlet business as legal heir. My father was the sole proprietor of the retail outlet and bank told that we cannot transfer his business bank account to my mother’s name so we closed that account and opened a new business account to continue the business operation. No one bothered till now but recently my mother received a notice from a person stating that one year ago he transferred 4 lakhs to my father’s retail outlet bank account as a loan from his bank account through NEFT or check. In the notice, he is demanding my mother to pay back that 4 lakhs otherwise he will file a case. I took the statements from the closed bank account and noticed that transactions for 4 lakhs happened from his account and we really don't have any signed document that my father took a loan. We normally receive the check and NEFT payments from our clients on weekly and monthly basis and don't know this is really a legitimate loan. What should we do now?