It seems this person ispretty new to computers and OS's. When you don't understand enough about disks, it can be hard for newusers to do stuff like partitions, and dual-boot.
If he has two hard-drives, the easiest for him to understand would just be to install Win98 SE on that.
Depending on what motherboard he/she has, probably dual-boot will be an option in their CMOS setup. When they reboot their computer, while it's loading the POST diagnositcs, hit DEL key to enter the CMOS configuration.
There, they'll need to become familiar with the motherboard's options. However, dealing with the CMOS is high risk to people who are newusers.