Definatley use a decent test meter which you can get a reading from and DONT go with that driver Chris, is your son-inlaw a spark? If so wouldnt do any harm to have him shadow you. Are you gona run the shower straight from the db on its own circuit?
Like has been said its most likely you are picking up an induced current, Dont go putting your trust in a neon driver though. If your son-inlaw is a trained electrician maybe get him to walk you through it