Yes, that's definitely a routing problem. The unfortunate customer who is dialing in is probably receiving all Internet-bound traffic from your network. I think your customer is using a Virtual Private Network (VPN), correct? If so, the following fix may help (for both Windows 2000 and 2003):
Open the Routing and Remote Access console. Expand your computer node. Right-click Ports and click Properties. Select the type of VPN connection, and click Configure. Deselect Demand-dial Routing Connections. Click OK until you return to the Routing and Remote Access console.
Right-click on your server node and select Properties. If Router is selected, deselect it, if possible. If you are using the server as a router, select LAN Routing Only instead of LAN and Demand-Dial Routing.
I Hope that helps!
This was first published in July 2003