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I don't think you need antivirus, I've certainly never used it on any Linux I've run.
I would agree with you, unless he is running WINE for known target applications (Office macro's, etc). Then maybe an anti-virus just in case. Not for his own infection, but so he can't pass on anything to a friend.
I don't have friends (aside from you guys) so I guess I won't bother. I'd rather spend the time backing up my data for the next 'clean install' I'll be performing in future.
Yup, I've been bitten hard by the Linux bug.