Download Virtualbox and be done.
VirtualBox does not, I repeat does not, support anything older then XP for Windows (with regard to consumer versions, which NT wasn't released to consumers until it merged with XP fully) with regard to their Guest Additions. They are clearly explicit with that.
You may have 10 or more crashes when you try to boot up Win 98 directly within Virtualbox. Or atleast I have. Also keep in mind, because you don't have Guest Additions, in order to share folders and get files in and out of the VM, have to setup things via IP and that is quite a bit more tricky with a VM then using the builtin Shared Folders that most VM software use. Not impossible, just not as quick or efficient.
I get around this by using Windows Vista in VirtualBox, within Vista, I run VirtualPC 2007 and then run Win 98. It runs at damn near native speeds once you get the VirtualPC drivers in Win 98 (until then it is awfully slow).
If he is going to do it through Virtualbox, he will have to either deal with those initial crashes before bootup and the trickier method of getting files in and out of the VM or do it like I do it, which will require more RAM and require using another OS within Virtualbox.
That's why I would suggest to see if he could get VirtualPC 2007 and to work on what I would imagine be a Win 10 machine. Maybe 8.1 (slim chance of a Win 7 machine, maybe if have a Win 7 machine and keep it off the WAN), but I'm playing the odds.