Uninstalling 2 instances on 1 node of a 2 node SQL Server 2012 failover cluster instance
When there is no instance on Node 2, does it become an Active Passive cluster?


Let me start by saying there is nothing like Active-Active cluster a more correct term which should be used is multi-instance cluster. When you have single instance of cluster it would simply be called as single instance failover cluster or just failover cluster instance (FCI), people do call it active passive but that is not correct. An active-passive cluster can have 2 instance on one node and other node having no instance which makes it look like active passive but actually it is multi instance failover cluster.


  Do I have to take the node offline before the uninstall? 


No, you do not have to take node offline.


  Can I get any links to uninstalling an instance?


It is in the same MSSQLTIPS link you have posted. And also note when you uninstall an instance you have to uninstall it from ALL nodes of the cluster. That is what the link says in line step 9. Please dont confuse by removing node and removing node from WSFC, its removing node from failover cluster instance(FCI).


  Am I right in understanding that you need the installation media to uninstall the instance? And that I cannot use uninstall programme method for this?


When you are removing FCI from Windows server cluster you have to run the setup file and select remove node from failover cluster like shown in the links you are already referring. If you do not have FCI but standalone instance of SQL Server you can go to add remove program and select uninstall, this will uninstall the SQl Server.


  So does it mean if I use the remove node option , it will remove only the instance BUT the node will still be there ?


Yep correct.