It depends how Parent A reacted. If it was with anger or intimidation then that's not on. But if it was her just telling the boy not to push, it's not nice, and if something is happening he doesn't like he should say stop, I don't see a problem. Kids need to know IMO that they can be held accountable (to a reasonable extent) for their actions by the entire community, not just mummy and daddy. It teaches them that the consequences of their actions reach further than their own mother or father watching them at the time. Pushing is the way this little boy dealt with being teased. Seems like he didn't have any other tools, and Parent A did suggest an alternative which he then used. I believe in the village approach to parenting. I would have been monitoring the situation a little more carefully, and as I said, as Parent B, would have walked over and started playing and chatting with all kids involved to see what was up. Also I would have taught my child other skills they can try when they're feeling out upon. As long as Parent A wasn't angry or threatening, I don't see a problem