How should his friend know what a harness is?

Of course everyone knows what a harness is. It is the vest they put on their dogs. Also, construction workers use one. You can buy one on Amazon.

As for a coding harness, I prefer the term agentic coding.

Having an experienced friend looking over your shoulder and taking notes of every step might help a bit, I reckon

I thought the idea was to test if she gets along without any help

I'm pretty sure the idea of the question "what harness did she use" was to ask the expert recounting this anecdote to include more detail, not suggest that he relay the question to his example friend.

The whole point of the anecdote was to showcase that it shouldn’t matter what harness she used.

That’s why the question only makes sense if it was relayed to the laywoman, in this case.