I think it's a very difficult ethical issue to solve. It depends on which are the characteristics that each of us considers "human". For instance, people with Down syndrome have an extra chromosome, but we all agree that they are "human" in all respects ... I hope nobody ever thinks of defining these people as "another species ".
Perhaps, in this case we should stop considering these cells "human" when they cease to be a good model for studies applied to human beings, regardless of the number of chromosomes.