He doesn't have a retail version of Tiger AFAIK but they'll work in any Tiger-compatible Mac. However, as of a few years ago (if that), Apple started putting in computer-specific versions of the OS with their corresponding Macs. This means that the iMac Restore DVD (or CD, I forget) has been specifically designed for the iMac hardware and is therefore only optimum when used on an iMac.
Technically, if he had a retail version of Tiger he could install it in both machines but this would still be illegal unless he had paid for the correct amount of licences (one extra in this case).