Sorry to be a contrarian but it would have annoyed the holy living hell out of me if someone butted in on a discussion/argument between myself and my child, besides sounds like the child was holding her own. Would any of you have wanted someone to butt in on the outside chance you were arguing with your kid who wanted to drink cows milk? I can see it now, "excuse me sir, but I've drank regular milk my entire life and I've never had any health issues". Not all omni parents are Neanderthals, many will actually listen to their kids arguments on such things, outsiders getting their dander up isn't going to help.
I also don't like people to get involved when uninvited.
I definitely see your point, and was thinking of that myself. That's why I made an effort to be brief and polite.
But my inner activist that I've been suppressing for a long time, managed to come to the surface.