Not that I have had experience with Octavias yet... But I have a 2006 Golf GTI and after owning it for about 12months the Airconditioner Compressor in it died.

The car had exactly the same symptoms as your dad's. Just blew warm air no matter what you tried.

It was a simple replacement that was done under warranty and it has been rock solid for the last 2yrs.

So potentially the AC compressor has died - but then again maybe something else.