I havent done this in years.... so take this with a grain of salt,, but its the general way to do it.

Each computer probably already has a network adapter, also known as Ethernet Cards. If not they'll need to add one.

If they don't have a network switch or network hub sitting around to connect a cat 5 ethernet patch cord from each computer to, which is probably the best way - but the least cost way they would use a Cat 5 ethernet "crossover cable" plugged one end into each of the computers network adapters.

Run the network wizzard on the computer with the modem and set up internet connection sharing. Which will offer to make a floppy to make it easy to configure the second/slave computer.

The computer with the modem will need to be booted into windows before the other can connect.

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Or they could go look around Apple's website and look at the Airport wifi routers,,, at least one, maybe all of those, has a dial up modem inside and then with the addition of wifi adapters both computers could be turned on and off and dial the modem anytime they wanted.

Thanks Mikie
She's up and running, well not very fast ( dialup).