Need legal opinion - sexual harassment?

Your friend should immediately call the police and file FIR against the girl and her family members-- even though the girl is minor in age but still she can be booked under criminal law for assisting her father and brother in criminal conspiracy under Section 120-B of the Indian Penal Code-- which is for "Criminal Conspiracy". -- If your friend will not file the FIR now then sooner or later either the girl and her family members will start blackmailing him for money or he will be booked by the girl under sexual harassment charges.