Harassed for giving illegal interest how can we take action

My father took loan from a khan three years ago, took about 3lacs rupees with an interest of 10% . He has no fixed income and he has pay him about 20k rupees every month.Now my father is broke and has no income, things have become really heated as he went bankrupt. But the khan keeps harrassing my father to pay him. The actual loan was of 3lacs. My father already paid 4lacs which was only the interest itself. But the khan was to keep takin money from him until he pays 3 Lacs at once. This has created chaos in my family and mother has severe depression. Family has fallen apart. I'm his son, and I just learned about this issue lately as my father was hiding it from us. Kindly advice what should I do?