Is she staying more than 90 days in a row? Or doing several visits which will add up to more than 90 days? If its the second, honestly I don't think there is much chance of anyone ever finding out, there are not really any checks within the Schengen zone and I wouldn't worry. (in fact if it's the second I don't think it's an issue at all, the issue is only really if she wants to stay more than 90 days in a row).