A bit of trivia for you Chris ;-)
If this is a dual boot setup with Win98 and XP and the person can boot into
XP they can open the partition that Windows 98 is on, locate the file called
system.1st - which is a hidden file in the Windows 98 root directory - and
open that file with notepad. Do a search for "productkey' and they will have
the number.
If they can't boot into XP they will need to boot with a Windows 98 Startup
Disk that contains Edit.exe. At the prompt change to the C drive (or
partition letter for Windows 98 setup). Then type ATTRIB -H *.* /S to make
all files unhidden. Switch back to A: and type EDIT. Then find and load
SYSTEM.1ST. Next do a search for productkey