People saying that £300 is incredibly expensive are partially correct, however they don't see the full picture. He walked up to the train station at a peak time, so of course the tickets are going to be very expensive. If he booked a few weeks in advance he would have been looking at closer the £30, than £300.
(Note: I think it has been said before but; If he had bought an 'Anytime single' to Newcastle, and then on the next day bought a 'Super Off-peak single' from Newcastle to London then he would have saved money. If anything the man on the ticket desk should be criticised for not telling him this.)
Also, just did a dummy day return booking on ba.com from Heathrow to Newcastle. It's currently 4am on Monday morning. For a return flight leaving at 7.20am and returning again tonight at either 6pm or 8.10pm the cost is.......... £ 338.64. Going straight to the airport may cost even more. From this perspective the fares are comparable/ slightly cheaper than by flying.