I'd go with a PCMCIA Ethernet card, they're easier to find drivers for and you keep the USB port open for future use, plus laptops that age usually have a dead battery so if you're gonna keep it plugged in you may as well go wired for the internet as well.
I would have to agree since the battery is dead that wired will be the way to go here. But odds are she might want to go wireless so I will offer both options to her. So it's safe to say that the card slots on this laptop are the same as those of today so that todays cards will work just fine as long as the proper drivers or in it?
Though I'd install Linux on that computer or at the very least Windows 2000, 98 is ancient, insecure, crash-prone, and hell to find drivers for, and the same goes for any browser still compatible with it.
You know, Linux may not be a bad idea. After all she seems to just want to use it for Internet and Linux with FireFox may just be what she wants. I will look into this as well.