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.
Interesting. Assuming the departure point was Central London I would allow 1 hour for the journey and 1 hour to check in and allow another 45 minutes to travel from Newcastle Airport to City Centre after landing. In this instance the train is not only faster but more comfortable and has a plug socket (and possibly a table) for a laptop. It's a wonder anyone uses the plane for this kind of journey.
The cheaper alternative is the coach but seating is subject to availability and reservation and all other coach travel related foibles apart (such as having to wear a seat belt, sharing a mucky loo, forced to sit next to someone smelly or with a crying baby) , your actual safety and journey experience is at the hands of someone being paid a few quid above minimum wage. How motivated do you think he/she will be to provide a smooth and safe journey? In my experience - not a lot. The 50k train driver qualified to drive you and hundreds of others at speeds up to 125 mph northwards - very much so. He/she will have been hand picked for that job that is seen as elite alongside Virgin and Eurostar.