Administrate on Behalf of Windows Azure Active Directory Module for PowerShell

I have a client that has granted me access to administrate on behalf of his business's Office 365 subscription. How do I log onto his account through the Windows Azure Active Directory Module for PowerShell, so that I can help him? When I log in with my username and password, I get my Office 365 subscription. I would rather not have to get his username and password. It's just not a very secure way of doing things.