Unless they just got on the train hoping to get away with it.
I don't think the people in question did this and credit to them for not doing so as
I positively hated it with the 'there's the situation YOU deal with it' attitude when I was a guard as it's invariably on a busy train with too many passengers as it already was!
Anyway the company's representative DID use their discretion -they considered what they could reasonably do and said NO.
Discretion isn't always saying YES, it's considering what options you have to resolve an issue whilst being able to justify that to your employer, shareholders and other customers.
Clearly allowing multiple people to just rock up and use a completely different service to the one originally booked (which is ostensibly discounted so much so the TOC could then balance out traffic flows) on 'token' fares a tiny fraction of more flexible tickets was not acceptable.
Frankly 'rip off Britain' annoys me since you can pick any episode at random and I guarantee at least half of the alleged 'rip-offs' are nothing of the sort, this devalues the entire programme IMHO.
Too many people feel they can shout scam/con/rip off (and some more blatant legal words for that matter) all over social media/general press and get away with it whilst unfairly besmirching the (good or otherwise) name of any size of company going about it's legally correct business and all because they (the customer) doesn't want to stick to their side of the contract.
Perhaps it's time someone was taken to court for libellous comments?