Just give the user permission to this mailbox....
Start Active Directory Users and Computers....
1.Right-click the user whose mailbox you want to give permissions to and choose Properties
2.On the Exchange Advanced tab, click Mailbox Rights.
3.Click Add, click the user or group who you want to have access to this mailbox, and then click OK
4.Be sure that the user or group is selected in the Name box
5.In the Permissions list, click Allow next to Full Mailbox Access, and then click OK.
One the user's computer that you want to access this account, just setup another profile on the users machine under Control Panel > Mail