To set an out of office message for someone else you will need give permission to the user you want to enable or modify the out of office message using the PowerShell extensions for Exchange At the power shell prompt issue the following command:
Add-MailboxPermission <user’s mailbox name> -AccessRights FullAccess -user <administrators account name>
Now you have issued the correct permissions to allow you to log into outlook web access (OWA) as the above administrator account and switch to the above user mailbox and set their out of office messages..
Using Internet explorer, go to https://<FQDN for your exchange server>/owa and login as the administrator's account you specified in the above PowerShell command.
Once you have logged in (may take a while if it hasn’t been used recently due to the way Exchange dominates a server) click the username with admin rights that you logged in with in the top right and type in the name of the user you want to set the out of office reply for. Once logged in as them, select options (top right), then select Out of office autoreply from the list on the left and complete as necessary.