0 Set Out Of Office For Another User

If you are administrator, or have access rights then log in Exchange WEB.
Usually : owaurl/ecp or owaurl/owa
Choose Options.
Select “Another User”.
Select the user.
Select “Tell people you are on vacation”
Organize Email
Automatic Replies
Type in the message and Save.

If there's no option for : “Tell people you are on vacation”
-> will be continued..