DOS printing

Hi,

I am trying to convince my Dad to buy a Mac but he runs this very old DOS program for his business. The program prints out invoices and proposals specific to his business. I was able to run the program perfectly on my mac using Boxer App and Dosbox. However, DOS only recognizes printers connected to LPT or COM ports. At the moment he has a PC that still has a Parallel port on it for the printer. Is there anyway that Snow Leopard or any third party software has the capablility of "tricking" the computer to make an LPT port? Or does anyone know if purchasing a USB to Serial port cable would do the trick? Thanks. I am sorry if I posted in the wrong area as well.