Although PCs running the Mac OS are completely unaffected by malware written for PCs running the Windows OS, they can transfer this malware without that user's knowledge. This is most likely how your little visitor gained entry to your PC.
Mac PCs can be vulnerable to malware, but only if that malware has been written to run on the Mac OS. If the malware is written for the Windows OS, it cannot affect the Mac. The worst it can do to the Mac is that it can pass it on to another PC.
Many computer users have difficulty in understanding the concept of malware, and fail to see that its various forms, whether virus, trojan horses, worms, spyware, adware, scare-ware, are no different than normal computer applications. Except for one huge difference; they usually are designed to be installed on computers without the user's informed consent, or to operate without the user's informed consent.
To prevent this happening again, you should let that user know he should use a virus-checker designed for the Mac OS. A good one is ClamXav (if this person uses OS X). It's free, does not need to be running constantly in the background and can be run just once every so often.