I'm curious, and I in no way am defending this player, but what exactly did he do? What I read was he was attacked, regardless of the reason, and responded. He "threatened" to attack back.
Every player makes decisions in this game. If you decide to attack, even for a raid village, or a chronos event, or to win rewards in a tournament, that decision has consequences. There are no rules that state if you attack during a raid village event that the other player can't attack you back. If you raid an emporia, there is no rule that says you can't attack a city in response. The part I have a problem with is the "threatening" and using alts to complete the threat. But you also have the ability to talk to allies, coa mates or even players not in your coalition looking to help.
This is the beauty and the curse of the game. The social messaging can be harmful and rude, or you can use the social messaging to build friendships and find players willing to defend and support.