I feel that you are possibly giving the OP's friend false hope here. The case in point is the fact that the passenger was found to be exiting Preston station without being able to present a valid ticket to show that she had paid her fare. She has admitted to the fact that there were facilities for her to purchase one at Chorley but that she did not do so. This can be proved.
Her defence is that she was given permission to board without a ticket by a member of staff at Chorley but crucially she was not given anything in writing to corroborate this, another point worth considering is - was the person authorising travel actually authorised to give this permission. It can be seen that the passenger will have great difficultly proving that authority was in fact given. There is, in my opinion, an outside chance that the member of staff concerned was indeed authorised to permit ticket-less travel, remembers the passenger concerned, and will confirm that he granted permission to travel - that said, I feel that that would be a forlorn hope and I fear this defence does not bear scrutiny.
As an aside, just who is in fact authorised under the byelaws to permit ticketless travel?
IMO, the defintion of 'authorised person' in railway byelaw 25(1):
authorised person means:
(i) a person acting in the course of his duties who:
(a) is an employee or agent of an Operator, or
(b) any other person authorised by an Operator, or
(ii) any constable, acting in the execution of his duties upon or in
connection with the railway;
is such that, as far as the byelaws are concerned, an 'on-duty' member of gateline staff would be deemed to be an authorised person for the purpose of permitting travel regardless of whether, in granting this authority, the staff member was acting in violation of company policy. After all, passengers ought to be able to rely on information given by rail staff.
That said, I understand that the burden of proof to show that permission was given, (and hence was acting lawfully), is on the passenger, on the balance of probabilities, and agree that it is most unlikely, based on what has been posted so far, that the OP's friend will be able to discharge this responsibility. If she had the name of the staff member who granted permission, then this would at least be a start, albeit that there is always the risk that the staff member would deny granting permission.
I also agree that, in general terms, it is prudent to get permission to start a journey without a ticket in writing
before actually boarding, although it is clearly too late for the OP's friend to do this!