Granted YBCG isn't the best place to do abinitio training, I believe your much better off at a CTAF.
Nevertheless, if ATC believe the instructor has been slack regarding the student's radio work then this should be something that is discussed between ATC and the training organisation/instructor away from the student. It's basic professional courtesy.
I had an interesting experience in a previous career in heavy motor vehicles. After I failed a driving test, entirely my fault, a heated verbal stouch ensued between the trainer and the tester due to previous history. Both reported the other, both were reprimanded and I was left with no respect for either.