Researchers might take different paths other than staying in academia after completing their PhD, postdoc or even later. This happens for a number of reasons. The most compelling of all is that there is not enough room for everyone, but it could happen that these people lose interest in research, find a good opportunity in the private sector, or (more often than we'd like) get burnt out.
For a typical researcher, there is a body of work that they have done and is susceptible to be published by people who they were collaborating with after they have left. However -beyond possible personal satisfaction- they do not have the motivation or time to publish this work anymore. This leads to poor communication between the authors that stay and the person that left, usually motivated by the latter not replying (perhaps reading) emails concerning the work or taking too long to do so. These people will usually be fine with having their names on a new publication even without the need to go through the manuscript before submission (this might also be true for reckless researchers in general, but that's another story...) because they might not be concerned about their (former) career in academia anymore.
How should this situation be dealt with? Is it ethical to submit a paper when a coauthor that left academia has not actively taken part in the preparation/proof reading of the manuscript, considering he/she has contributed significantly to the work?