Not given possession on time by the builder

Dear Experts, My father is over 60 years of age and a retired govt. servant and my mother is also in the old age. My father got a by-pass surgery few years back and he is still recovering from it. adding to it he is diabetic and suffering from blood pressure issues. My mother is also suffering from various health issues which comes at an old age. I am the only son in the family and earning my bread in Delhi. My parents are putting up in Kolkata. My father booked a flat in kolkata when he got retired. He invested his maximum share of funds in that. The builder promised to give the possession in June 2016 but till now he is not able to give the possession as the builder is going through a financial crisis. Not only this the builder is not giving any promised time now as upto when he could deliver the flat to us. Another problem is that my parents are living in a rented flat whose agreement is going to finish this month and the owner of the flat is not ready to re-new the agreement due to some family reasons. Now at this health condition and at this old age my parents are feeling hopeless when the 80% of the amount of the flat has been given to the builder which is a handsome amount as for a middle class family. The builder is not giving the money back and neither he is able to deliver the flat. My parents are feeling very much harassed and I am really very much concerned about them. Please suggest some good ideas or ways by which my parents could come out of this situation as soon as possible.