One of my advisors suddenly passed away while I was in graduate school. We had some discussions and ideas about future publications, but he passed away before any of the work was completed. When the work was finally completed and published, I and my co-authors were therefore presented with an ethical dilemma about how best to acknowledge his contributions to the ideas behind the paper. Should we list him as a co-author? Put him in the acknowledgements? Listing him as an author would give credit for the original idea, however, we would have no way of knowing if he actually approved of—and would want his name attached to—our methods and writing.
In the end my co-authors and I decided to list him as a co-author with a footnote stating that he passed away before publication.
I'm interested to hear from others who have been in similar situations and/or suggestions on what constitutes "co-authorship" when one of one's collaborators passes away before the publication or work is complete.