I suspect most experienced instructors would have stepped in before the swear words started!
Ah, so true....
I am not an instructor, just a pilot. But, I know some things about flying and landing. There have been several occasions when I have simply been a right seat pax, thinking the guy on the left knew what he was doing ('cause he owned this fancy plane). That assumption has nearly got me killed a few times, when things started going wrong, and left it way too late before I stepped in.
Instead of my early on saying "...have you noticed... could happen shortly", I was trying to be polite and quiet. But, no, he had not noticed. Then it gets too far down the wrong road.... Um, either this guy is going to have to be really good to get us out of this, or I'm about to have to do it myself.... Oh geeze....!!
I always got it back, but sometimes it was not pretty, nor polite. A sudden, completely unbriefed "!!I have control!!" Happily, no one ever objected, as they saw the need too, but it certainly could have introduced a personal conflict, and fairly so.
So, personally, I need more training as to how and when to jump in - so the candidate pilot gets the most of the learning experience, but everything stays safe. In two pilot test flying, we brief the flight, and that works well. But nearly always, I'm flying with a real pro, so these things are not issues. If the pilot is not a pro, that becomes evident in the briefing, so problems are resolved before the flight begins.
But checking out another pilot of mystery skills and recency can be hazardous. There is a balance between respecting their skills (whatever they are, once you figure it), pointing out where they are lacking,
and, accepting where they show you that
you are lacking!
Tailwheel training is on the scarier side of this sort of training, as things begin to happen very quickly, with little time or space to correct.