I'm definately with the controller on this one.
As controllers we undergo countless hours of training (200h + in sim, 600h+ OJT) and gain tons of experience when working... A bucketload of "SAFETY" is molded to our backbones, and while working this is REALLY the number one priority.
Every single day we take lots of operational decisions, and every single one is passed by our "safety filter" before it's put into action. I am certain that the ATCO in question did exactly this before letting the kid "parrot" for a few moments.
I've listened to the audio-clip and it does not appear to be a busy situation. Regarding the juridical aspect I have no clue, but I am 100% certain that it was 100 % SAFE all the way. The instructions from the kid could not be misunderstood in any way, and the ATCO in question had him on a short leash, being able to take over in an instant if anything out of the ordinary happened.
my 2 c.