A friend of mine wrote a research proposal a few years ago. When he had 80% of it done, he pitched the idea to a colleague who joined and wrote the extra 20%. The proposal got rejected. A few years later the friend is trying to write a different proposal that has some aspects (but not content) in common with the old proposal. However, the colleague who co-wrote the old proposal will not respond to multiple emails requesting him to join the new proposal -- they probably lost interest but for some reason won't come out to say so.
Lets assume the proposal goes ahead to submission without involving the earlier collaborator. If the proposal were to get funded, isn't there a risk, that the earlier collaborator could turn around and claim they somehow influenced the direction of the new proposal yet is not credited on the proposal? How in general do people move on after a joint proposal gets rejected; and some members won't be interested in joining offshoots of the old proposal. Of course in this case there is the added challenge of the earlier collaborator refusing to come out and make it clear that he is uninterested. How does one avoid the potential accusations of being unethical?