You can either talk the user through this, or setup a new outlook profile to open their mailbox and set it yourself using the GUI – both are time consuming.
This simple powershell command allows you to set permissions with ease:
add-mailboxfolderpermission -identity USERNAME:\calendar -user "Username of person who needs access" -accessrights reviewer
The Identity switch needs to be the username of the mailbox which you are giving access TO, the user switch is the user you are giving access FROM.
The accessrights switch is the level of access you wish to grant the user, the link below lists some additional switches you can use:
http://technet.microsoft.com/en-us/library/dd298062.aspx
