OK ... when you use one method or the other to register the purchase you must use the same installation method everytime so it is assiciated with the same account. So if you use the downloaded option he also must use the download it is just that simple and the way the rules were written.
And the fine print in the rules says you can install the program on up to 5 computers that you personally own ... so do you own the computer that your brother is using ? If not then I would not squawk to loudly about how it needs to be installed.
