If it is a DOS program, get a copy of DOSEMU and then use FreeDOS or some other DOS clone to run inside of DOSEMU. Then he still will have the DOS Command Line, but under Linux using DOS emulation. No Windows required to run DOSEMU.

[link|http://www.dosemu.org/|[link|http://www.dosemu.org/|http://www.dosemu.org/]]

[link|http://www.freedos.org/|[link|http://www.freedos.org/|http://www.freedos.org/]]

Play around with them at home first, see if it will run the DOS based accounting software. Then apt-get-install DOSEMU and FreeDOS on Linux at work.