Each disk that ships with a Mac (yours in question) is made to work with the type of Mac it ships with. For example, if you have the first gen iMac C2D the install disk that ships with it will only work with that exact Mac in that exact generation. It wouldn't even work if that Macs were exactly the same except yours is Rev A and his Mac is Rev B.
Also you should know that it violates the EULA. Your friend needs to buy his own copy of Leopard.