Bank did not do settlement of loan and closed account

In 2000 my dad had taken a salary loan from a cooperative bank he was working in. Due to personal reasons, he decided to leave job in 2002. Per discussion with his manager over phone in 2002, he had sent his resignation and application to credit proceeds from PF, gratutity and security deposit to loan account and close off the loan account which bank did not do in 2002. In 2012, he came to know about not closure of loan account from his colleague. So in he went to bank and bank had agreed to do settlement of loan by paying off some additional amount which he did. This communication was verbal. Trusting the bank, my dad did not ask for closure letter to bank. Now suddenly in last week after almost 6 years, bank official called him up and said there is a huge outstanding of Rs. 6 lacs in his loan account. Bank had never called him in last 6 years for the same. My dad is also retired now and cannot afford to pay such a huge amount. What should he do now? Can he file complaint against bank in consumer court and how is the overall process.