A manager can post a message to any user and send as private. You should see a public/private option when you click on the Post link on the discussion home page.

As I look at the code it looks like a member cannot send a private message to anyone, unless private messages are enabled for the entire forum. So.... I will add code so if a user clicks on the send message to manager link on the discussion home page then the user will have the option to send it private even if private messages are not enabled..