Q: How do I remotely view a Remote Desktop session in Windows Server 2012?

A: Windows Server 2008 R2, 2008, and 2003 offered administrators the ability to shadow a user's session to a remote desktop session host and see what the user was seeing and even interact. This feature has been removed in Windows Server 2012 Remote Desktop Services. If you need a similar capability, you should use the Windows Remote Assistance feature.

The FAQ "Q: How can I remotely enable Remote Desktop Services on a remote server?" might also be of passing interest, though it doesn't deal with your need to now use Windows Remote Assistance.