How can I communicate with wordpress users avoiding their email?

Hi,

My client will have to be able to allow users to register to their wordpress site but wants to be able to allow the user to communicate directly with them and visa versa without email being involved. A sort of private messaging system, but the user must not be able to communicate with any other user – simply the admin. The admin also wants the option of transferring files to that user privately.

What’s the best way to do this? I had considered implementing a buddypress system and allowing the formation of a group of 2 to allow the private communication, but buddypress comes with a lot of stuff I won’t need.
Is there a collaboration type plugin which would allow this communication and sharing of documents? Any help or advice would be appreciated! Thanks