I don't see what's happening in the screens, but it doesn't look like the person is breaking the doors down (particularly since you said you managed to close the gates behind him, which would mean that they were still intact) and it doesn't say that he opened the gates. Before you ask, I do not have logs for this case as of yet, I shall leave this open until I do have them, but for now, I shall speculate as to him being ignorant over intentionally griefing;
If he did this through naivety rather than to intentionally cause the defenders to lose, then it's not a bannable offence, if there was a discussion, I assume you mentioned politely why it's not a normal practice...
In this case, his ignorance is bliss, I'm not one to punish someone for something that they didn't realise. If you see him again, tell him nicely that it's not a good idea and people will become upset.
Cheers.