He cant install 98 with XP home edition becaue neither one of those have a boot manager. He would need a program like Partition Magic or Partition Commander to make it work . If he has XP Pro it does have a boot manager but installing 98 after XP might rewrite the master boot record and he couldnt boot into XP.
Are you saying that if you have Win 98 instaled first and then install XP home edition, it will not dual boot because XP home does not have that option, only XP Pro?