Firstly, is there any evidence anyone was unable to board as a result?As I understand it, other TOCs are obliged to help where they reasonably can.
If helping means their train becomes overcrowded at departure to the point that their own core passengers at intermediate stations are unable to board, leaving them with an extended wait for the next train on a relatively low frequency route, would this be considered as a reasonable reason as to why they couldn't help (or could limit the number of passengers they were able to help)?
Secondly, when disruptive incidents occur, there should not be any preference given to a company's "own customers"; the same standards should be given to everyone.
Thirdly, this is an entirely moot point as the customers had permission to board, from an authorised person; any charge to those customers made by Scotrail is unlawful, regardless of anything else anyone wants to debate!
(The concept of a company's "own customers" isn't one that I embrace, though I understand anyone who is a fan of a disjoined privatised railway system where each company does its own thing may disagree with me, but that's a whole new topic for a new thread!)
If anyone doesn't like the fact that the customers were lawfully travelling, and wants to make some proposal to change the rules, or attempt to justify the unlawful actions of the Scotrail guard, feel free to create a new thread; it doesn't belong in this thread, which is specifically to give advice to the original poster in respect of their friend's journey.