Results matter. If the player is producing poor results for an extended time then perhaps the private instruction isn't working or the player doesn't understand it. If I see a pretty "standard" mechanical flaw that I can't imagine is being taught on purpose then I'll ask the player about it and give some small cues. If it seems to be a flaw in their approach or what they are being taught to do then I'll speak to the parents and see if it's something they want me to work on or leave it in the instructor's hands. I'm not going to fight what they are being taught, but the parents have to understand that poor results lead to less playing time.