Mother in law filed case as heir of my late wife

My friend's ,,,, father in law before his death gave us one house to live in it and also wrote in his will that he gives a house to him and his wife . his wife and father in law died in car accident . Now his mother in law has filled case for possession of house he is living in . they have no children , mother in law says that she is heir to her daughter and owner of the house .. What to do ,, please help