Macs will not automatically mount NTFS drives that weren't unmounted cleanly. In other words, if he yanked the plug without properly ejecting the drive first, it won't work. I'm guessing that's what the problem is.

With MacFUSE it is possible to force it to mount a drive that wasn't cleanly unmounted from the command line, but the easy solution is to plug the drive back into a Windows computer, run scandisk on it, and then eject it properly.