If you can look at her in a slightly pitying way, then look around at the others with the same expression, that will help seal the deal. 
I wish I’d thought of that. Several years ago, I was in a winery in Arizona with my mom. I was having a very interesting conversation with the sommelier about wine and the guy who was making the wine I was drinking.
My mother, who had no idea that I know anything about such matters, turned to the sommelier and said, “Don’t listen to her – she’s drunk!”
I was mortified, and so was the sommelier. We sort of blinked at each other, I smiled brightly and changed the subject as if my mother had not even spoken. I was around 35 at the time. I’d had maybe two sips of 1 oz. pours. Mom just felt intimidated because she realized I had expertise in something that she knew nothing about, so she felt compelled to diminish me in front of the sommelier to bring me back down a notch to the compliant little 7-year-old she apparently thinks I still am.
I waited until we finished the tasting and were safely in the parking lot before I ripped into mom. “Don’t ever treat me like a stupid little kid again, especially in front of strangers in public. That was humiliating!”
My mother immediately contracted amnesia and first tried to pretend she hadn’t said that, and then tried to pretend she didn’t remember saying it. I know her short-term is like Swiss cheese, but even she can remember something she said in front of witnesses less than an hour before.
She never took responsibility for that and I gave up trying to prove I’m an adult. I think I actually proved it at the beach (see my previous post in this thread).