Physical punishment is really abuse. There's nothing wrong with calling out abuse of any kind. If a child is misbehaving, it's actually helpful to bring that to their parents attention. Be careful to address the child's behavior. For example, my son has ADHD and a few years ago, my wife was picking him up from his first day at school. The teacher told my wife, "Your son's a real space cadet". That was totally unacceptable. If she had simply described his behavior, that would have been different

If you make assumptions about how they are parenting based on how the child is behaving and try to give them direction in that context, you're in dangerous waters. That applies to you whether you have a children or not. You may not know what is going on with the child. The only time I feel like non-parents should "shut up" is when I hear one say, "If I had a child, I would never let them...." Yes, yes, if you had a child, but you don't...so shut up.