Well, it doesn't mean that it
isn't his host that's being banned, that's just a possible problem. With the error 'host infected...', it would seem to me that that's the result of some trojan scan done by the server upon connecting, so he might want to check his computer, using
www.nohack.net or #Nohack on irc.dal.net as starting points, or else google the name of the trojan mentioned, as well (as you said) as checking the network's information about k-lines, if any is given.
