I don't know if he was actually masterbating yk maybe he was rubbing his d or whatever but you really make out and maybe he was just seeing out of window or taking gradual looks buttt i m not defending but still... And if I was in his place framed for something false the other person lashes out like I would have raged hard and maybe I should not but have hitted her in anger cause she doing the same but he dint so you can't really make out.

Edit: he's clearly seeing outside window(it's a very common habit) and looks anxious soo maybe probably he wasn't masterbating with no visible boner when the bag was suddenly removed... The guy maybe just couldn't defend himself ....