I would not recommend doing that under any circumstances. While he may not have been punished for it yet, going duo top/duo mid because he doesn't want to play the role he was filled into is a punishable offense. That is absolutely griefing. Not to mention, it's risking the entire game by leaving an adc to fight a 2v1 lane bot, and stealing half the exp and farm from whatever lane he goes to sit in. That is quite rude to the rest of the team, and since he mutes them, he wouldn't even know if they are asking him repeatedly to go back to his lane and stop trolling.
With the way the IFS works now, it's much harder to prove the non-verbal infractions. Basically, the IFS is a bot that scans games after a report flags it, which is easy to do in the case of harassment and verbal abuse. Keywords are picked up by the bot. But it's a bit harder for a system to pick up things like this, where a player is refusing to play their given role and refusing to have any discussion with the team. It doesn't mean that a player won't be punished for it eventually though, it just takes longer for it to happen.
If you are familiar with the case of the Support Smite Singed, then you would know that he received hundreds of reports, if not more, for his actions in game. While the original response from Riot Support was amended, the follow-up clarified that his actions did fall under the category of insufficient communication with the team. It was not enough to simply tell the team what he wanted to do. Part of playing as a team meant being willing to work as a team, and being willing to make compromises when the team brought up significant concerns regarding his strategy.
By muting his team immediately, your brother is not allowing for any communication or discussion, and the fact that he is doing it immediately after saying he's breaking the meta means he knows that it's going to piss his team off. Which kinda implies that he knows he's griefing his team. He may have just been lucky and not been punished yet, but I would strongly advise against doing this yourself. While the verbal infractions start off with chat restrictions, the non-verbal ones do not always. Some of them only have two tiers of punishment: 14 day ban, and permanent suspension.