Yu-Na not talking to Brian in person to announce the split doesn't surprise me. When I taught the piano I had a few pupils stop, and even when it was expected they seemed to find telling me a very traumatic experience. There were plenty of tears and embarassment. Often, even the older pupils would come accompanied with their parents to bear the bad news, and some would simply send their parents instead.
In my relatively short career I never experienced anyone asking to change teachers, but I can imagine that is even more uncomfortable.
It seems Orser has little experience in that department if he expected Yu-Na to phone him and tell him in person... That pretty much never happens! It seems parents are nearly always in the frontline in these circumstances, acting as a cushion of protection for their child. Even if their child is an adult!
This is not so say Yu-Na acted with class and dignity during the whole episode, far from it.
But I can see that Orser hinting that Yu-Na's mum was the cause of the split was not a good move.