The air is still warm because it's warm outside. You'll probably find come winter time that you only get warm air out of one vent, or no vents at all.
Maybe either that or casting sand from when the engine block was manufactured that remained in the engine block and eventually found its way into the heater matrix over time.
All sorted bud it was heater matrix and did it all myself in under 2 hours and literally bought the new heater matrix denso for £72.00. Its not even hard just have to fiddle about with it.
When we took the old one off it was rusty from inside you could see and then when cleaning out you could see the rust it had developed over time
Seems quite short... you'll likely find that the cooling system is full of air I'm afraid.
All sorted mate had been heater matrix , but there was no physical leak just devorting coolant like nothing.
Cleared the air let it run with cap off and it took the coolant then topped it up and now just holding it perfect mate
Unfortunately the cooling system on this car is incredibly complicated. I suspect you'll have the symptoms of an air lock come winter time.
Just curious why you're certain he has air in the system?
Having seen the correct procedure, I find it incredibly unlikely that a two hour job without diagnostic equipment was sufficient to do the job. Only time will tell.
As I said above....it was bad enough on older VAG cars..the circuit/flow diagrams/schematics on these newer VAG engines give me a headache...anyway they'll find out later on..& spend ages chasing it around!