Can anyone please help me out and indicate the mistake in this exchange?
“Is John ill? He’s lost a lot of weight.” “Yes, he is rather slender these days, isn’t he?”
I am not so sure about the way the second sentence is punctuated.
Punctuation is a matter of style and preference and it largely depends on which style book you or your editor use. However, using a comma is not as much confusing as other punctuation marks.
According to the linked Commas: Quick Rules by Purdue University:
Use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause... Common introductory words that should be followed by a comma include yes, however, well.
Use a comma near the end of a sentence to separate contrasted coordinate elements or to indicate a distinct pause or shift.
The punctuation in your second sentence is fine
Yes, he is rather slender these days, isn’t he?