To add a little bit of chapter and verse to the good advice that has already been given, the reason why lots of people here think you could argue the point and win is because the National Rail Conditions of Travel say at Condition 6.3 that
The National Conditions of Travel here use much the same words as byelaw 18(3)(iii).
So since your daughter's boyfriend was told that he could travel, everything's fine? Well, no. If the matter ended up in court, the court would have to decide if they believed his story. Strictly, it is up to the prosecution (the railway) to prove their side, but ultimately it would come down to the court having to decide which was the truth - your daughter's boyfriend's story ('the man in the ticket office said it was OK just to get on the train') or the railway clerk's story ('I don't remember ever saying to anyone that they could hop on the train'). Of course, if the railway clerk confirms that he gave permission to travel, at that point you are home and dry.
Exactly right. As Yorkie and others have said, if permission was given to board and pay on train then no offence was committed at that point. The strict liability nature of Byelaw 18.1 has not been compromised because there is an absolute exemption provided by 18.3
If it can be proven that a traveller subsequently actively avoided a further opportunity to pay that would present a different picture, but we're assuming that isn't the case here. For example it is very common for ticketless traveller's on ATW to arrive at say Cardiff Central and offer to pay only from the last stop of the particular train that they travelled on rather than the further distant station where they actually boarded. I'm not suggesting for one moment that this happened in this case, but it does happen on a regular basis, even when they had been given permission to pay on train at that more distant starting station.
The major problem for revenue protection staff and the Company is the level of inconsistency in application of their own rules by other staff
Recent announcements about enforcing their 'Buy Before You Board' policy confirm that is ATW stance where ticket offices and both cash & card payment TVMs are provided. That position is supported by the Byelaw.
Announcements to that effect in the press by Directors and prominent signs at stations warning the policy have been a regular feature of ATW over recent years, but it seems from many complaints that some of their staff regularly ignore their own company rules (occasionally with very good reason) and the general inconsistency creates conflict.
There will always be genuine occasions when it has to be accepted that a traveller can be given permission to board without a ticket and pay at the first available opportunity, but the widespread abuse through 'pay only if challenged' has resulted in where things are now on ATW and unless there is clear evidence that permission has been given, conflict is likely to arise.
If I were the traveller in the incident reported here I'd certainly go to Caerphilly ticket office, speak with the person concerned and seek a note from that clerk confirming that permission was given verbally.
ATW will be able to confirm who was on duty at any given time and check internally although they will not be likely to provide the name & details to a member of the public.