Presumably it was a virus on his PC while the N900 was plugged in as a mass storage device, rather than a virus running on the N900 itself. Lots of PC malware likes to spread via USB flash drives etc.
yup most of them just like to autorun so as soon as you plug it in they create a new autorun.inf file with instructions to run themselves as soon as the usb is plugged elsewhere
i got rid of the virus t.com or somthing using x-term but the autorun.inf i could not recover