We don't know the context, but given NK said she only told one person in her life (Charlotte) I suspect it was in relation to her telling Charlotte that Chris was "separated" with children. Her comment was that "She's supportive, but it came up, because something similar happened when she was a kid" (paraphrasing), then followed by the "I don't care, I am going to keep seeing you" and then followed up with "are we bad people?". Totally my opinion, but that's what I am guessing - keen to hear theories of what others think that exchange might be about.
There is simply no evidence that I saw proving NK told CW to work on his marriage. It's just her word. Again please do correct me if I am wrong as it would change my opinion!