What if this happens:
Say I contacted a researcher R in my field about particular question I had. They answered the question in a private email to me. Their sketch of the argument was about 5 sentences long, and it can be turned into a rigorous proof. I really like the result. However, R has been unresponsive for 2 months and has not replied to any of my subsequent emails where I make various offers and seek their advice.
Would it be unethical to include their argument as a small part of a publication I'm preparing, assuming I give credit to R? What constitutes proper credit in this case? A sentence in the introduction when I first mention the result? A sentence preceding the proof? In Acknowledgements?