dollar for your thoughts?

I go to Dollarama to buy pens, jars, and Bubblemint gum.
Last week, as I wandered down the kitchen aisle, eyeing up non-Tupperware tupperware, a mother and her young son (maybe aged ten or eleven?) trailed behind me. They were looking for a lunchbox he could take to school.
He points at one and she says "Don't be stupid, that's too small, your lunch won't fit in that one."
"I think it will," he says.
"Oh my god, no it won't. Are you retarded?"
She chooses a larger one.
"We need a new mop too.." she says, as they reach the non-Swiffer swiffers.
"No mom, we don't need the whole thing, we just need the top..."
"What are you talking about?"
"Look, mom, we just need one of these refill things, not the whole thing.."
"Oh my god, are you retarded?! You're driving me crazy today.."
"Mom, look, you just buy the top and it clicks onto the handle we already have at home!"
"..Oh.. okay then."
"And stop calling me retarded.."
"I didn't call you retarded... but stop saying retarded things."

It was the saddest.
I really regret not saying something.
Not to her. Who am I to give parenting advice? I don't have any kids, Besides,  she's probably still a good mom. I mean, at the very least, she makes sure he goes to school and makes sure he has lunch, and those count for something.
But I wish I'd told him he sounds like a smart kid and never to listen to anyone who tries to make him feel stupid, especially people who use "retarded" as an insult and don't even understand the concept of replacement swiffer attachments.