I agree that sending a NC letter to the two PA partners may be unnecessary IF they REALLY are already no contact. Sleeping dogs.
Depending on the depth of the EA, a NC letter may or may not be appropriate....but leaving it "open-ended" as he did wasn't right. Perhaps the best thing, if you trust him, is for him to never respond to her again.
Someone else may have better advice. My WH was unable to send a NC letter because the OW was his employee and I said "nothing in writing, you dumbass".