Go to Custom Recipient on the File menu and create a new account (all this within Exchange Admin). Call this new account, the person's name with an addition, something like John Smith Forward (it is just a pointer). You will be asked to input the forwarding address. This will create an account with a globe as its icon (more importantly, it will add this new account to the Gloabal Address list). Now, inside the person's mailbox properties, go to Delivery Options, click modify and choose John Smith Forward as the choice. The mail should start forwarding. Now go into the mailbox properties of John Smith Forward and click the Advanced tab, click Hide from Global address list. Nobody will see this account. You can view these hidden accounts by clicking on Hidden Recipients in the View on the tool bar.
Good luck