When giving a master class or private lesson to people who are not your regular students, I feel it is very important to not do or say anything that might come across as disrespectful to those students' teachers and/or their work with those students.
Also, I have heard many stories from students going to workshops and seminars in other countries outside their own country. This may seem farther afield, but I think the principle still applies. In this case, I still feel it is important to have a sense of respect for the other various ways and styles of playing other than one's own.
For example, if I am giving a master class in a place where the teaching style and the equipment used are different to my own, I don't suggest that someone should get a new horn or say to them, "Get rid of that instrument because it is junk and the the tone is terrible!" like I have heard some people say in their master classes.
There is not one way to play nor one instrument to play. This, I think, is the kind of attitude that breeds a frozen, one-dimensinal viewpoint that does not breathe or let in new light. Why can we not embrace the differences as all a part of the marvelous spectrum that life and art is? If we were all meant to be exactly the same, then why weren't we all born in the same country, all with exactly the same genetics?
I also do not think it is right, in most cases, to change someone's embouchure at a summer festival if you are not, as a teacher, going to be around to see it through. It is one thing to make suggestions but to insist on a dramatic physical or style change, in a short period of time, actually could be very harmful for some people.
To give some advice and offer some ideas, in the musical or playing realms, that add to the person's musical development is one thing. To degrade and insist that there is only one instrument and one way to play and/or only one interpretation of a piece of music and to attempt to impose significant changes on a student with whom you do not regularly work, is insensitive, arrogant and unrealistic--in my opinion.