Basically here's what happens. If it's not through PMs and they keep quiet before we arrive, we have nothing to go off of except the other guy insulting the person who supposedly insulted his dead mother. Now unless he or by-standers have videos/pictures of the other guy insulting his dead mother or we can find it in the PM history (Last 10 or so PMs, so it's likely to pop up) then he gets off free while dead-mother guy gets punishment for breaking the rules.

tl;dr

Dead-mother guy was the only one caught badmouthing while the guy who was said to be insulting his dead mother was caught doing nothing and so dead-mother gets punished while other is free to go.