After spending many hours on the issue, he finally tracked it down. When he originally bought his printer, the sales rep had also sold him an Epson Parallel to USB converter cable. This appears to have been (see the comments!) what was causing the OS X printing problems. As soon as he replaced it with a straight USB cable, everything worked fine.
So if your Epson won't print, check your cable -- and maybe just plug and unplug it, per the comments below. If that fails, it may be time to try a new cable.

