I wouldn't have said anything. It's not worth it. She's not going to learn anything. Seriously, the woman's daughter is missing and she's wasting time pointing fingers? You will never get through to people like that. Don't waste your words or your anger on them.
Sometimes I find it best just to laugh at them. No one likes to be laughed at, and it makes me feel better. (Not that you should laugh at someone with a missing child). And I'm not implying that I don't get angry...much to the contrary...but after a little vent I'm all good!
As a side note...my kids don't always behave in public. In fact, I would say I'm lucky if it's fifty/fifty! I certainly do not expect others to take care of them or watch them. And I'm always trying, but I don't think hiding out in my house will teach them anything! (And no, they don't steal food!) I guess I just understand how it feels for people to expect you to stay home because your children aren't perfect.