If the emails all are Klez Infected the person likely does not know he is sending them, and stands a good chance that he is not sending them after all. Once infected with the Klez virus it will copy all adrress in the email client to do 2 things, email it itself to all of them, and to rename the email header to appear as if from another person. I work a t a Univiersity that has yet to block the virus on the mian servers so studnet who get infected are constantly spreading it unknowingly. Just becasue you have a anti virus program installed does not help, for in order for it to be effective you have to keep it up to date, weekly if not daily. Most viruses are out for weeks even months before anti virus companies are made aware of them, and able to disinfect them.
With your situation, anyone with that person in their address book, could be the infected computer, yes this includes yourself. I say this simply ebcasue on a dialy absis I have to delete over 30 infected emails, with usualy 3-5 syaing they are from me in the headers. I know I'm not infected, simply because the attachemnts inlcude can't be opened.
Send and email his way jsut to give him a heqads up, otherwise not only him but everyone else is you 2 have in common become the vivitms in this case