Misbehaviour in flight

I was on a flight from Bangalore to Delhi at night 11. I witnessed misbehaviour with a fellow passenger, so he booked a window seat in advance. He was young, around 23M, let's call him Ashish. When he arrived at his seat, a woman around 40W with passenger 25M was seated on his seat. He politely told her that it was his seat. She kindly answered that she wanted a seat with her passenger and asked him to sit far behind (not a window seat). Ashish reminded her that it's not her seat and he booked it in advance then the lady shouted "Didn't you hear me dead I am not getting up, don't disturb me " Then her passenger started barking like hell "Stop involved personal life and Now shut up and go" Ashish didn't get a chance to explain to the air hostess. The air hostess asked to get away, and when he explained that it was his seat. The hostess didn't say a word and continued to push toward the back. He repeatedly asked, shouting that it was his seat. Who was she(the lady 40 W) to tell him what to do? Now, one listened to him, and everyone just said. I wanted to help but I also lost hope. After the flight takeoff, the lady passed near me, and Ashish was beside me. She just smiled a little and said, "Waste fellows". If this happened to me what can I do 1) what to do with a passenger and a hostess does nothing? What can he do? 2) where can I complain against her and the air hostess 3) What can I do if similar things happen on a train where the passenger has taken all the space under the seat and is disrespecting me