Simply enabling DHCP won't enable any routing, NAT, or other gateway services on the Win2k machine. All it will potentially allow the Win2k machine to do is to provide IP addresses and some other information to the iBook.
Instead, with the default tools he'd need to set up Internet Connection Sharing (ICS), though that may not be possible depending on his setup (you need two NICs). (NAT would possibly work, as would bridging connections if his ISP supports handing out multiple IPs, though I don't believe this is supported natively until WinXP/Win2k3 server)
Paul, when you say you have a "network point", what do you mean? Is it a router/firewall? A wireless access point? A switch or hub?
It almost sounds like you may not need to use the Win2k machine at all - if the Win2k machine connects to the Internet through the router/firewall, it's very possible that the firewall will serve as the iBook's gateway as well.
If you have to run special software on your PC to "log in" to the DSL network, see if you can configure your firewall to log in automatically. If the router/firewall has multiple ports, you should be able to plug both your PC and your iBook into it, set both to obtain their IP addresses via DHCP, and both should be online.