Heh.
DS was 16 last month and STILL never shuts the fuck up!
A few years ago his friends and other kids at his school had a scale for how much people talked and the ultimate was "...talking more than <DS' name>".
He does have ADHD and is medicated, but as soon as it became obvious at around 3-4 that he was a massive gobshite I drilled it into him that you take turns at talking and that you don't use a loud/outdoor voice.
I do feel for that mum - I've been there (still am) and it really is exhausting - but when out and about I just kept reminding him:
"When you're too loud, other people can't enjoy their food/hear themselves talk/concentrate on reading..."
"I have to think now,"
"My ears are really tired now'"
And repeat.
AND repeat.
Sometimes, though, you literally do just give up and stare into space.
My finest moment, which I still recall fondly, was telling him that his rugby gumshield needed to be in his mouth for ten minutes to set properly and that he had to stay quiet for that exact length of time.
Fucking silence for ten glorious minutes. It said on the box you only needed two minutes, but you take your victories when you can.
Then there was the time we paid him £1 per minute of silence.
We were waiting for the teenage monosyllabic grunting phase, but no. It's not happening. He's a bit quieter when he's medicated, but he doesn't take the Ritalin evenings/weekends/holidays, so we get the full force.