VPN Server behind cable modem

I have a client who has a cable modem. He wants to be able to access his NT server (behind the cable modem and a router) from a remote office. My understanding is that the VPN server must have a valid Internet IP address. In this case, the modem hasbeen assigned the IP address, and the router and computers behind it are on a 192 network. Has anyone found a way to make this work?