Most likely, there are other computers infected with a virus. Your friend's e-mail address is probably in the address book of the infected machines.

Have your friend head to http://housecall.trendmicro.com/ for a free online virus checkup.
You could also pick up norton antivirus 2004 which has e-mail virus checking automatically built in.

As for filtering these e-mails, just delete them... It's typically good practice to never accept (or open) an attachment unless you (the user) are expecting it. You can also probably set the e-mail client to automatically decline downloading any attachments.

Happy virus hunting