First, I want to say that I very much respect welsummerchicks opinions. She gives good advice very often on the boards and really does seem to think out her responses before typing them up. I am also friends with the OP. I don't think I've let that bias my response here. Just putting it out there, though, so you can judge any bias for yourself.
I understand that welsummer is coming into this with professional experience dealing in not just conflict resolution, but the resolution of severe conflicts - the type that can be not only dangerous, but deadly. So, her advice is given based on that. I think it should be respected. Her approach is probably something that we should all consider of great value. Any action that we take in a situation like this could escalate a tense situation. Our actions, whatever they might be, could help trigger a person into responding in a way that is dangerous and/or deadly.
I also understand the OP's response, as I stated earlier. I truly believe that as adults we must stand up to behavior that is abusive, especially when the actions are committed by people with power, as this woman has been described, directed at people who are "weak" in comparison- especially those unable to stand up for themselves for some reason. Thankfully, the vast majority of times when we do this, we are not in a situation where it is likely to lead to danger. However, we should recognize that there is always some risk.
Let me illustrate this with something personal -- and I don't often get this personal online.
I grew up in a situation that was not ideal. I learned coping techniques that included NOT standing up to people who acted inappropriately. I learned to say "I'm sorry" even when I'd done nothing wrong. It was part of a set of self-protective mechanisms geared at avoiding escalating conflict. This was not just at home. It was necessary at school too. Anyway, as a result, I did not learn OTHER coping mechanisms that are needed in life. I learned how to avoid problems, sure. I could de-escalate a situation like a pro. The outward appearance was one of calm, a perfect child/teen, etc. It all looks hunky-dory to an observer. But, there are other consequences that are not visible to people looking on from outside. I still deal with those to this day.
In this case, sonew, the OP, taught her daughter that she did not have to accept blame for things she did not do. She let her see that she WOULD stand up for her and would protect her from stronger (in any sense of the word) people who were abusive. Her daughter did not internalize blame for things she did not do. Does that mean she may have been 100% perfect sitting back there? No. She's a teen out with her friends, after all. Expecting her to act perfectly is ridiculous. I seriously doubt the adults in the audience were all silent & acting like angels. If they were, well it's unlike any junior high or high school concerts I've been to in the last ten years,including a couple where half the adults were carrying on full fledged conversations, including on their cell phones.
Were there other possible approaches that would have been usable in this situation? Sure. We can usually look back on any situation and see ways we could have done better in that particular scenario. I'm sure that the OP looks back and sees words she said that she might change, or something else she might have added. However, she also may have taught her daughter a lesson that people should not use their position of power and authority (even if only perceived authority due to social position) to dominate someone who is "weaker." By standing up to a bully, she also may have helped teach her daughter that this (bullying of a less powerful person) would be an inappropriate way for HER to act in the future.
Could this conflict have escalated? Let's be honest. Yes. It could have. The woman could have gone berserk. She could have raged and pulled out a weapon. Or, she could store up the hatred and it could explode later. There could be effects in the future, directed at the OP or her daughter. We'll hope that isn't the case.
In this case, the OP has to ultimately judge for herself if it was worth the risk of a potentially greater conflict. Assessing risks and acting in response to them is something we all do constantly. Some of us are just more aware of it than others and most of the time, our assessment and decision is subconscious. In most cases, our actions aren't going to result in violence. However, there is always a chance. I don't know about others, but I can NOT live my life in constant fear that every word I say or look I give someone will result in violence. This is how children / teens (and even adults) in gang ridden neighborhoods live. This is how children living in abusive households live. This is how women in a domestic violence situations live. I personally "choose" not to live the rest of my life that way. (That said, I also have relatives with whom I go into automatic conflict resolution mode to keep a situation from going haywire...it's an adaptive technique that I've chosen to use with them quite regularly.)
That said, we DO have to recognize when a situation is getting out of hand and take steps to turn it the other way. I believe that welsummer has lots of experience and practical advice that could help us all with this.
The only people meandering were the folks like me taking their chicldren home after they completed their band portion of the show-since our kids aren't in chorus, we don't stay for the whole show-I could have but I was whipped and it didnt get over until after 10pm and I live 1/2 away:-( I have a feeling that woman would ahve waited until after the show and done the same thing except in front of a much larger crowd! I realy am glad not that many people were around-the bigger the crowd the bigger the drama and uuuuugh...what happened was big enough in my book 
