I was not there so I can't say anything to that case but in general I think you can not proof abuse by whisper.
I see the point that a GM has to see what is happening if he is going to ban someone for something. The GM can't see the whisper.
The problem is, that the chat is not logged. Not at all. That is a problem exactly for such things. Would it be logged (even if only for 12 hours at a time) you could have a proof.
The best thing (slightly off-topic): Some people who seem calm, nice, fair, adult, who will tell you to behave and then whisper you with the worst curse words. Found that funny when that happened to me
About religions: There are people who take religious terms as abusive. I would watch my language when i curse. sounds stupid but using political or religious words might easily be taken very offensive.
About offensive names: I think there are two kinds.
The one that is clearly offensive like "Nigga", "Hitler", or even whole sentences, and no matter in what language. For example lately there was someone sitting in town "Du verfickter Penner" (You Duck up bum) which is a very bad way of cursing someone in german. And clearly directed at everyone who reads and understands it.
The other kind is just a very annoying name but not really an offence. Something like "imawuss", "A**hole".
Also there are names you only see they are offensive at the second look. Like some time ago there was char running around "sinep" read backwards....
@MrWho: Someone asking for IP-check or IP-ban is something natural for everyone who already was GM, Moderator, Administrator somewhere or simply knows the common technical possibilities in a community or a game.