Win98 doesn't recognize the NTFS file system, and it never will. There is nothing you can do about that.
Try using a Win2K startup disk, if available. You can convert the file system to FAT or FAT32 with partition magic (or similar), but I'm not sure what the results would be. Been along time since I used NT, but if I recall correctly, it does recognize FAT and FAT32 filesystems. You do NOT want to copy 98 files to an NT system anyway. That would be a sure way to kill whatever else might be left on the drive.
The easiest way is to just reinstall NT (he does do backups.....right?). Once you get a clean install of NT (or whatever OS), get an image of the drive. Makes for pretty painless reinstalls if ever needed.