Minecraft Message Program?

Hey guys, this recently happened to my friend:
He's a big PvP'er and angered a lot of people on the server. Shortly after, a message appeared in the middle of his screen, in the Minecraft font, saying YOUR COMPUTER IS INFECTED. He got pretty freaked out, and wanted my help to figure out how this happened. At the time, he had the Xenon client installed, although this has no broadcast system that I am aware of. Later, after he was freaking out on the server, another message appeared: Its a joke, calm your tits. Being somewhat ready for it, he was able to react fast enough to snap a quick screenshot:


If anyone has seen ANYTHING like this before, please tell me. We both agree this would have very many uses, for trolling and other things.