Without having to login to their account/mailbox, use Exchange commandlets to get/set their Out of Office.
To view current Out of Office settings:
Get-MailboxAutoReplyConfiguration -Identity DOMAIN\Username
Link to Technet: http://technet.microsoft.com/en-GB/library/dd638081(v=exchg.141).aspx
To set properties for a users Out of Office:
Set-MailboxAutoReplyConfiguration -Identity DOMAIN\Username -AutoReplyState enabled -InternalMessage "I am on holiday until 01/08/14. Please call Joe on Ext 123 in my absence" -ExternalMessage "I am on holiday until 01/08/14. Please call the sales department on 0800 xxxxxx in my absence"
Link to Technet: http://technet.microsoft.com/en-us/library/dd638217(v=exchg.141).aspx
Re-run the Get-MailboxAutoReplyConfiguration command to check the changes have been made correctly.