You've managed to overlook the most important part of what Bookem told us ... "The pc is now infected and the source appears to be the emails from the mac user."
Many common PC viruses circulating today use what is called "email spoofing". When the virus infects a PC, it harvests email addresses and starts sending itself out. Each copy of itself that it sends out has a fake sender address picked more or less randomly from the email addresses it has harvested.
This means if you recieve a virus generated email that appears to be the emails from the mac user then it most likely hasn't come from that user after all, but from a virus infected PC.
And yes, all the current Mac anti-virus software recognises viruses from all platforms, however that won't help if the virus had never come into contact with the Mac anyway!