He needs a new bench with one of those contoured seats, I'd arch my back a -tiny- bit when benching 80kg, since I got my new bench i dont arch at all now =)
Also doesnt help if you got no weight on the leg extension at the front of your bench(if it has one)
I didnt get to see how bad this guy was arching though
i think he might be mentally crippled already, at least a little bit.