It is possible. If the user shares his calendar with the person who is trying to reach the calendar. This even works outside the company. Logon to OWA with your own logon credentials. It will look something like this:
This is the silent url, after logging on to OWA you can put this after the previous line:
Replace %username% with your own name or the name of the person whom’s calendar you would like to view. The syntax of the %username% is the same as logging on to the OWA.
First logon to OWA! => You will see your inbox. Add %username%/calendar to the URL: