House construction

Hi all, One of my friend started house construction last June 2016. Agreement signed by his mother without knowledge engineer constructed extra square feet Nov 2016 he demanded 100000 extra. My friend not accepted but engineer volunteer stopped house construction. There is no way he accepted and signed to agree give 100000 but needs complete as per agreement again they stopped work. Till date not completed. Value of work pending Rs. 70000. My friend asked engineer he not willing to complete work. My friend ready to complete work and less the amount to his pending work. But engineer not accept anything. As per agreement six month. Now more than one year complete he not hand over key. He locked door and keeping himself and blackmailing money and today he telling he will break window and doors. What do now?. How get keys from. My friend gave 98% amount. Only two 2% only pending. He not complete work and asking full money. What we do now. Please help us..