If a teacher sees a student doing something wrong (posture, bad fingering, etc), should he or she say something?

My friend has had his teacher criticized by another teacher (who has never met the teacher, only heard of him). The "advice" was un-solicitated. The other teacher heard him play and started criticizing. He then preached about how the first teacher was not teaching the fundamentals that he should, etc...

Well, my friend is confused now because he liked his first teacher and really thought of him as an expert. Now, he's wondering if he should switch to the new teacher...who seems to know so much. So does a teacher have the right to criticize another teacher?

No. It's highly unprofessional. Perhaps the current teacher is choosing to work on other skills at this time. If it is a huge concern, the student should raise the question with the current teacher.

Yes, the student's welfare is paramount.

Well, does he literally have the right, of course he does. But it certainly makes him look like a sleazeball. I wouldn't go to study with a teacher who did that.

This would at most entitle the critic to attempt to teach the student something, not to badmouth the student's current teacher. (Unless current teacher is a convicted criminal etc.)

I wouldn't hesitate to say a student was being taught poorly if tension issues were being left unaddressed.

It depends on the situation. If I'm judging a contest or evaluating the student for an exam, of course it's my job to say something.

But if the situation is unsolicited, then it's best to keep my big mouth shut. I've had other teachers and even non-teachers criticize me, so I'm used to the territory.

You know what my philosophy is on tension, but there are other "schools" of piano-playing out there (I mean, way, way out there). Is it really my place to criticize other schools of piano-playing? Of course I have my strong opinions, but I'd rather keep them to myself and my students and my students' parents.

If asked? Or without being asked?

There'd be a conversation like:

"You've got a lot of tension there in your wrist. What does your teacher say about that?"

"Nothing"

"That's not good."

You're happy when other teachers (of varying quality) do the same with your students then?

Well it wouldn't happen would it:

"You've got a lot of tension there in your wrist. What does your teacher say about that?"

"A lot."

"That's good."

Why the need for ridiculous assumptions? Other teachers are going to focus on their own pet topics, not on yours.

Pet topic!??

Instead of "That's not good," try saying, "You might want to discuss that with your teacher" or "Perhaps at your next lesson you can ask your teacher about that."

But I wouldn't even start such a conversation.

No, I might add "Are you sure?" before the "That's no good."

Imagine a car leaving the garage after a service billowing smoke. "What does your mechanic say about that?" "Nothing" - you really gonna suggest they go back for a little chat?

Your post essentially assumed that no person anywhere is going to find anything to criticize in any of your students (which is quite different from actually finding anything wrong). Even if you were some sort of idealized perfect teacher, stupid-and-opinionated teachers (of which there is certainly no shortage) would find loads of things to (wrongly) criticize, including introducing your students to all the misguided notions that you (rightly) never think to mention.

Eradicating unnecessary tension is hardly '[my] pet topic'! - which was my point.

Eradicating unnecessary tension is also my "pet topic," but I don't go around telling other people that their technique is "wrong"!!

There's this kid who wins competitions left and right in Southern California, who plays with the tensest shoulders imaginable. I know of his teacher, but I'm not friends with her, so I keep my mouth shut. Maybe if I'm friends with the kid's teacher, then I'd go straight to the teacher and mention something. I don't want to make it look like the teacher has missed something. Who knows? Maybe the teacher has spent years correcting this shoulder problem, but the problem is beyond fixing? And if he's playing well, maybe it's not that big a problem!

I can't see the point in saying anything to teachers who allow poor technique. They're often as not the cause of the problem.

If everybody agreed what "poor technique" was, and precisely what and where it came from, ...

But they don't. Being ready to criticize the teaching of other teachers, to their students, requires being equally open to letting other teachers unfairly criticize you to your students - because face it, unfairly is how it's going to happen. I don't think that's a good deal for anybody.

If we're not all agreed that unnecessary tension is the major element of poor technique then I'm stumped!

Well, stumped you are, then. I, for one, strongly disagree.

"Eradicating tension." Really !

Back to OP's question. I made the mistake of questioning my sister-in-law about her son's lessons. It was interpreted as criticism. I've learned my lesson.

My job as aunt is to sit there and be impressed.

That is the only situation in which I would ever have felt comfortable offering an opinion, and now I won't even do that.

Does a teacher have the right to criticize another teacher? Yes.

Does a teacher have an obligation to to criticize another teacher? No.

Should a teacher criticize another teacher? Probably not, unless your students all walk on water, and you yourself walk six inches above the water.

This is such a fine line to walk on. The hard part is knowing what the teacher is teaching vs. what the student is doing. Offering criticisms of the student, however, is certainly OK. Then it's up to the student to decide if those criticisms are correct or not, and to reflect on if their teacher was telling them to do those things, telling them not to do those things, or not mentioning it at all.

If another teacher told my student to do something that I've been working with them on and it's not clicking with the student, then sometimes hearing it from a 3rd party finally sinks in.

Now if the criticizing teacher knows for a fact that another teacher is teaching certain things that are detrimental for the student, then it's really that teacher's obligation to say something.

Many people love their teachers for reasons other than what they get taught, and in many cases this is fine. However, when it clouds their judgment to realize that they're not progressing, they are suffering injury, etc., due to poor teaching then they need to have someone open their eyes.

That is what it sounds like happened to the OP's friend. Now that friend has to decide if what was said was true or not, and if so, ask their teacher why they weren't working on these things.

A teacher once came up to me and my son after a public recital. She started criticizing his technique in a certain piece. We nodded politely and listened. My son's teacher happened to be within earshot. She came over and pulled the other teacher aside. She very politely let her have it.

The other teacher was way out of line. She was certainly entitled to her opinion. In that sense she had the "right" to do whatever she pleased. Her manner of expressing it, without knowing a thing about context, reflected a certain "attitude." My son's teacher, who is quite a professional, had no problem with another teacher talking to her privately about what she observed, but to walk up to my son that way was distinctly unprofessional and completely inappropriate in the context of a recital of this sort. This was not a master class situation.

BTW, KBK seems quite happy with that attitude. It's a presumptuousness that most people I know find very off-putting.

Agreed. In this context or any other.

(edit) Just want to add, this has occurred to my daughter as well, and the only result is that she is made uncomfortable in the presence of the, shall I say, new teacher. It is a bestial act towards a child, who has a relationship of confidence and complicity with a teacher.

Oh dear, that doesn't sound so good. Sorry, but I can't just look on. I suppose I'm just a have-a-go-hero when a person's piano-life is in danger.