I'll stick my neck out here, Ken, and offer a guess. My neck is brass, so I can handle the flak!
First thought: without any fact to base this on, mind - I reckon you have been playing so well, for so long, that you have probably forgotten the crushing, paralysing apprehension that grips you when trying to play in front of others the first few times. The guy who was playing was obviously one of the weaker, less confident members of Donny's group. He's playing single note, and he doesn't define the notes as well as he could, it's all one long glissando.
Now, this being first and foremost an instruction and mutual support group, Donny, the group leader, throws in some support to flesh out his effort, to make it sound good. Which he's entitled to do: his ball, his rules. I think it improved the offering, and I'm guessing the player appreciated the gesture of support.
It's not the same as an accomplished player doing a recital, with everyone else keeping a respectful hush. So to take the view that it was bad manners (certainly implied) seems slightly to miss the point of what they were trying to do. If Donny chimed in with some harmonics while I was playing, I'd be happy.
Sent with fingers firmly crossed. Just my two penn'orth...