From a business perspective, I'd say go with the Blackberry. For a normal person, I'd say iPhone.
His work probably uses Exchange or the ActiveSync protocol, so it will work with the iPhone. Unfortunately, there is no central place for calendar mail messages etc.
I've been trying to get Outlook 2010 to work with the iPhone, but it fails as I assume Outlook 2010 doesn't register itself as main main handler or iTunes is just oblivious to it.