I was going to suggest that. But... then I realized that his original message implies that his ex-leader is a plunderer which means he doesn't convoy and thus....

He could, in theory, plunder his other former mates, but it's not really their fault.

I guess he could join the group that is most frequently plundered by his former leader and diligently provide support such that it makes his leader fail at plundering more. That's kinda weak thou and won't have full coverage...

So... I guess there is no game mechanic that he can take advantage of. So spreading the news to get people to quit that group and eventually collapse it, as the leader is obviously problematic, is about the best he can do. But people aren't just going to quit groups w/o perspective of a new one, one that at least have almost as high group skill level as the old one. So he need to find and arrange for transfer targets for his former mates if this is the case.