B is absolutly free to defend himself against any aggressor, that includes C, which is an alt-char of A.
Tricky part here is, if B would be engaging C first, which then falls under mentioned rule, but could also be seen as malicious reporting as A knowing B would be there joined the fight anyways and then reported B for attacking him on C and thus breaking rule 5.8.
It's called common courtesy. If you join a fight willingly, accept the consequences.
In your given example, B sure as hell is free to defend himself... and to answer your question... neither A nor C did break the rules, as they've been the winning side anyways in the fight against B, and the rule itself just covers the losing party.
