• Family friend building contractor harassing senior citizen for more payment

Hi Experts,

My father, however without my knowledge and with consent of my younger and elder brother decided to construct a house on the land purchased by him few years ago. My father in law is also contractor to make small houses but my brothers decided not to hire him for constructing the house instead they hired a new bee contractor who unfortunately is their friend. Initially that's so called friend contractor has promised to construct the house in 15 Lakhs (paid by my father in cheques) and my pa didn't sign any contract thinking him to be a family friend but after 70% work is done he wanted more money which my father initially told no but that contractor somehow used his charm and agreed my father for paying another 3.5 lakhs. This time my father has taken I think an affidavit for this payment atleast. Two months passed and I think house was about 90% complete and then suddenly he stopped working. I think 3 months from now he would have stopped working and gave various reasons to my father like demonetization, my labours went home etc.. my father trusted his words and was ok even after paying extra 3.5 lakhs even though it was tough for him to arrange the same since he is retired government employee and worked at the post of grinder. Also within this period he was asking money by giving silly arguments like give me my transportation amount (in which he goes there in his car with her wife) for site visits and monitoring and was asking 65 thousand extra which my father denied, since no good or damn builder or contractor will ask money to go to his work. Yesterday came a shocking moment when this guy sent notice asking for 6.2 lakhs of pending installment and my father is completely disheartened and already a patient of disease having symptoms like Alzheimer. Now only today my father called my father in law for help, when he checked the notice, seems like it is sent by an advocate without any court stamp, asking to pay the amount in 15 days. 
Could you guys give suggestion how to go about this issue? 
Also he kept his locks on the house and says every time key is with my painter or some one else? Since as per land and house document my father is owner of property can we remove those locks and get in? any advice would be greatly appreciated?

I live away from my home town because of my job and not at all sad that my father didn't even tell me about the house construction and all, I just love my father since it's because of his sacrifices only I was able to study and now able to earn a living. Please give your valuable advice

Thanks in advance,
Nitin