By a weird coincidence, I actually once did see a middle eastern boy berating and demeaning his mother in an airport, saying things that I would never have said to my mother at that age. Of course I often rebelled against my mom's authority, but this sounded more like he was refusing to acknowledge her authority at all. He was treating her like a servant, not a mother. I was appalled, but I didn't say anything, and neither did anyone else. I was afraid that some adult male figure, or perhaps even the woman herself, would get in my face and say "Don't tell me how to raise my kid!"

As I think about Rebecca's case now, I think the best supportive action would be to speak directly to the passenger, saying "You owe her an apology", then walking away. Good to think about it now, as it might increase my chances of doing the right thing in the future.