Generally, the technique done properly works with or without friends, at least in my experience. When I've had problems with kotegaeshi (and I've had many, some in public), slapping them in the butt didn't help.
And if I want to take someone's balance with atemi, I can think of better places to slap them (other than the butt).
But none of that really helps my **kotegaeshi**. It might make me a better slapper though...
RT
PS this is not a rant against atemi...I'm just trying to focus on the technique for a moment...