Send an email to sales@stardock.com

As mavx says, you're going to need to contact stardock sales.

Word of advice though, don't give out email addresses any more than you have to its asking for trouble. Or spam email.

does it matter to what email its attached? i mean its only about who controls the key right? or am i totally missing something.

As the serial key is the only way to access updates/redownload the game its worth keeping on top of it. If its registered to his email then he has no trouble managing it, if his mum closed her email account for some reason he might find he loses access and has to re-purchase the game. Best keep things neat.