It sounds as if your client wants to use the Internet as the backbone for dialing into a presumably secure site.
This can be done through different means:
1: You can still use the dial in method if money is a restriction, to make this safe I would recommend that the dial up be made and then use PPTP (Point To Point Tunneling Protocol), if you have an NT server you can configure it to do so.
2: If money is not so much of a restriction then you may want to think about installing CISCO routers at either end and creating your own VPN (Virtual Private Network).
Both PPTP and VPNs work in the same way, they both provide a means of transporting normally unsupported protocols through the internet e.g. NetBEUI or IPX/SPX (the internet only uses TCP/IP), they also encript the data from end point to endpoint, thus ensuring data integrity.
Establishing these types of connections would take to long to explain so what I recommend is, for configuring PPTP on an NT server (or Win2k) for a client to access visit the knowledge base within the Microsoft website. You may also want to buy or at least read a VPN configuration book to understand the technology before attempting to implement it. CISCO sell books on exactly this topic and will provide you with detailed technology aspects aswell as configuration options when creating VPNs through CISCO routers.
Hope this points you in the right direction.
