Can one be penalized or blamed for an unfinished task in a company if he is no longer employed there?

My cousin resigned from work months ago, and the company he used to work for is still blaming him now for some back jobs and stuff that was not completed when he was still working for them. They want him to asnwer questions etc. They were undermanned then, that's why he gave up, since he couldn't handle it anymore. Is this fair or correct for him? I mean making him involved now that he is no longer their employee? He did not even have a contract with them.