I think the problem is that he needs to go from USB (from the Modem) to Ethernet (on the NIC). Most seem to go from Cat5 female to USB male.
One possible suggestion it to find a USB router. Your modem would plug into the router, and you would have RJ45 ports going to your lan. It should be relatively inexpensive.
Another suggestion is to use that system as a Gateway.
I'm not sure that this will work, as I've never tried to use USB for networking, but it might be worth a try.
If you're on Win XP:
Hook up the modem to the PC, via USB. Hook up the rest of your network to the to the Ethernet
Then run the network setup wizard.
You would choose "Other PC's connect through this PC to get to the internet" ( something close anyway, don't really remember)
It would have to be on to allow the others to access the internet, of course.
Once again I haven't tried this with your particular configuration though. I've only tried it with two NICs and never tried it on Win98
Good luck