Should someone else tell them?
Sure, the CFI or Head of Training is the one is responsible for what goes on, and the one who should decide whether a word in the shell-like of the instructor/pilot is necessary. Not having seen the manoeuvres, I couldn't comment whether they were "dangerous" or risky. It would depend on who was on board, what level of experience they have and what the objective of the flight was. If it was PPL training, Ex 23 Advanced take offs and landings / Ex 21 Quickstops, it might have been justified - whether they were performed minimising time in the avoid curve is another question.
Pirouettes are an exception - not part of the syllabus and not easy to justify why exactly you were doing them "at the time when the engine failed Your Honour" - except to say that they are fun and sell PPL training quite well. Hands up any instructor who has never done one just for the fun of it! They can also be a demonstration of how tail rotor thrust affects power available and therefore why they can be a dangerous thing to do.
Next question is, if this were a trial lesson, are those sort of manoeuvres that the punter has signed up for? Probably not, I've changed my mind over the years and cut out things like EOL's from the T/L format. They used to be almost standard, but the punter has signed up for "air experience" not simulated emergency training.
The avoid curve is just that - to be avoided if possible, but if we always followed the recommended take off profile, we might as well fly planks and only ever operate airfield to airfield.
My advice would be, unless there is a reason to be in the avoid curve, stay out of it! If you do have to go into it, minimise the time that you are exposed.
Nevertheless, pilots have a duty to share safety concerns, so talk to the pilot involved and find out what they were doing and why. If such behaviour is habitual and unjustified, then you may want to make sure that their CFI/ HoT is aware of your concerns.
I know of 2 instructors who became more and more cavalier until they had accidents. In both cases the worsening trend went unreported to the CFI until it was too late and it is a matter of regret that concerns were not raised earlier that may have avoided 2 needless deaths.