plagued with viruses

His email address is being targeted because his email address is in the address book on an email client on an infected computer. He doesn't recognize the sender because the "From:" address is most likely being spoofed. To find out where the email is actually coming from you'll have to look at the email headers and find out the host that originally sent the email.

From our personal experiences,the people that do a lot of mass forwarding of e-mails seem to be targeted at a higher rate. (Chain mail, Sales people etc.)
Today's viruses can steal addresses, e-mail identities and passwords, so who they come from hardly ever are the original submitter.
If you have an in house e-mail server you should have some type of Active Virus Defense as blocking only attachements may not give you sufficient guard in the future, it's getting rough out there.
I would suggest changing her address if she is really getting blasted.