I'm not as erudite as most here, so please excuse the long post.
IANAL but have 'supported' (done it for them) someone in reversing their criminal record for not having a student railcard when buying a student ticket. Their life was seriously impacted and damaged by it being on their DBS.
It's taken two years, and the railway company itself, never once spoke to them. (or me) It's intentional.
Honestly, MH or no MH, if he did have an appropriate railcard when he brought the ticket, he'd be crazy to plead guilty, and crazy to not go to court and defend his clean legal status.
Do make sure this is 100% factual and there's no situation where the railcard was not yet valid? (young people sometimes tell parents silly lies not realizing consequences, to cover embarrassment) then forget MN attitudes about abandoning offspring on their 18th, and go with him.
Pleading guilty will result in a fine and far more importantly him receiving a criminal record which will stay on an enhanced DBS for 11years (and on record for 100 years) which can affect his career prospects, travel, and immigration status.
In a legal (or caring) career, having a criminal record is a really bad start to life and normally reserved to mark out a specific class as less worthy of trustworthy employment. You/he need to kick back.
He's being charged under railway byelaws which is why people think they have no teeth or consequences.
They are very wrong, and confusing them with fixed fare penalties under bylaws, where you pay a direct fine and no criminal record if paid.
Then government brought in single justice procedures, which changed everything in reality, raising both revenue and criminal conviction rates.
The byelaws he's charged under are backed by legislation, and he's being prosecuted in the magistrates' court for what has moved on from 'give us proof or money and we'll leave you alone', to now an 'actual criminal offense', and is about to get a criminal conviction if you/he don't fight against it.
If unlucky he can expect one/two wasted days turning up for 'case management' hearings - if lucky it could get dismissed at that point or sent back for reconsideration of either dismissal, or a fine, but no conviction. If not, then a further day in court where he answers to the charges, which if he has used a legitimate railcard to purchase his ticket, he will walk out with dismissed as not guilty as charged, and it ends there.
He's studying law, he needs to see this as a useful experience of challenging how law works and doesn't, and the opportunity to be around those being subjected to it, rightly or wrongly, and the hopefully soon to be discredited often flawed 'single justice procedures.' A bit of direct experience could be part of a future essay for him.
Doubt he's being done for 'failure to produce the railcard' and actual charge is traveling / attempting to travel without previously paying the fare (correct fare in his case because he couldn't show entitlement to a student railcard discount) and or having intent to avoid payment of the fare or entering the transport system without a valid ticket to travel. He's a law student, make him do the homework on the actual charges!
Your Ds can do this, and if needs be, he can ask for his mum to speak for him. (Entirely up to the magistrates, but they generally don't like dealing only with a confused, tearful, breaking young person, if there's a calm competent adult with them who can focus on the law, as well as circumstance.)
If it's Cross Country Investigations the train company has outsourced SJP to, they're notably poor at paperwork and understanding that offenses must be correctly charged.
I'm not a lawyer or legally trained, just court experienced from coming from a difficult background, but frankly ran rings round them using common sense and breaking down whether the charge was appropriate for the alleged offense.
Law's a factual thing, even if it can be open to interpretation. Facts, are facts.
Ten to one he's being done for traveling / attempting to travel without previously paying his fare- which he's not guilty of, or having intent to avoid payment of his fare- also not guilty of, and he has the evidence to prove it. (assuming the railcard valid for the time and day) So why mess up life by pleading guilty?
(He should have sent a picture of the railcard btw, not just the number.
Always send maximum evidence, and easily understood and dealable with by whoever's on the lowest pay grade, not what requires additional / more skilled work.)