Since Scotland is part of the UK the 'authorities' (e.g. the UK Border Agency) have no interest whatsoever in anyone's movement between England and Scotland. It's no different taking a plane between London and Edinburgh than taking a bus between Brighton and Hove, or a train between Swindon and Reading. No passport or ID is required.
Airlines require ID for security purposes but all airlines accept a valid passport, irrespective of the issuing country. So your friend will have no problem flying to Scotland from England (or from any other part of the UK).
Chris



