He doesn't need to boot from it. If he can load the disks, and install the platforms, he can tell LILO or Grub to boot from the drive, though it's always better to have an internal.
Distant, boot from the Slack disk into the terminal, login as root, and type lsusb
That will show, if detected, what your HD is. Then mount it with... the mount command, to some folder in /mnt
Then run fdisk or cfdisk with as (c)fdisk /mnt/<mount point and you should be able to setup the partitions.
I'm not sure about Fedora..