Would someone with knowledge in fluid mechanics please help me in understanding this man's argument for why dp/dy, the pressure gradient in the boundary layer, must equal zero? I would greatly appreciate this as I have watched his video numerous times trying to follow his argument. His reasoning is at the 2:28 mark in the video.
https://www.youtube.com/watch?v=h3RuIhR6TTU&index=5&list=PLPhpE8md80HOfBesybSGUcba2YWce4_fz