If your friend leaves the residence / home that you share, then one of you will lose your right to the program / shared license.
The license is for up to 5 people who share the same home / physical space as their primary residence (on personal / home computers - not remote office computers).
If your friend leaves, and you keep the original disks, then your friend loses all legal right to use the program.
If your friend takes the original, then you lose all rights to use the program.
Basically, up to five computers / users in the same residence. No sharing with friends or family who live somewhere else.
That said, I have successfully created a full working backup copy of the entire Leopard DVD using Windows. Windows was able to see the Mac and the Windows sections of the DVD and created a single large image that contained both.
These types of disks are "Hybrid" disks. They are common and have been around for as long as we've had CD's.
Many companies publish Mac and Windows versions of their programs on the same disk. The PC reads the PC section, the Mac reads the Mac section. Both machines ignore / don't see the section intended for the other machine type.
How did you create a backup of the DVD? Any chance of a break down on the steps?