Sweet God, am I reading right that you just sat and watched as a grown man screamed and shouted at a small boy, then locked him out in the rain and refused to let him inside for three quarters of an hour?
Even when the Dad was cold enough to come out, remove the toy the child was playing with, and then go back inside?
That's not a heat-of-the-moment ill-judgement, OP, that's intentionally calculated punishment, and it's not normal parenting, at all. I'm wincing at just the thought of someone treating my DD like that.
That said, my DD would be utterly distressed if I did that to her, an the fact that this child isn't says he's used to this - even worse still.
Please call the Police, OP. I understand why you wouldn't confront, but you have to do something. Or the school, I guess. If you're sure which one. It's better than nothing, but please do something!