The traditional method of opening Windows Task Manager, going to the Users tab, right clicking the user, and clicking Remote Control is no longer and option on Windows Server 2012 R2.
To shadow a session in Windows Server 2012 R2, you must use the "mstsc" command with the /shadow switch. First, open Windows Task Manager and go to the Users tab. Find the ID of the user you wish to shadow and remember this number. Then, from RUN or a Command Prompt, type “mstsc /shadow:<session id>”. The user will be prompted to allow you to shadow their session. This will work on Remote Desktop and normal servers.
If the server is a Remote Desktop Server, you can use Server Manager to shadow the session. Go To Remote Desktop Services, then Collections, and find the Connections window. Right click the user and click Shadow. The user will be prompted to allow you to shadow their session.