So why doesn't the card holder get off the train, go to the inside of the barrier, and meet the friend at the barrier - hence being able to show the railcard if required?
I don't see the need for the cardholder to exit the barrier and re-enter, and hence BoJ restrictions shouldn't be relevant.
I think AlterEgo's concern was that the friend joining part-way may be "starting short" if they have the same ticket as the OP. There's no need for them to be - they would probably be better off buying a ticket from their origin (unless they plan to travel back to OP's station on the return).
It's sensible to check, but most likely there wouldn't be any BoJ restrictions; most flows in the SE have only CDRs available (generally without BoJ restriction). I'm not sure about SWT's Super Off Peak tickets.
Unless they really have to, it would be an unnecessary hassle to meet the friend at the barrier - there might only be one train an hour after all!
It's all about the definition of 'travel'.
There may be problems getting past a gateline as you need to be able to show (when asked) a valid ticket and any supporting documentation such as railcards. Personally I think that the ticket isn't valid without the railcard holder being present.
Certainly, the ticket isn't valid without the Railcard. The
T&Cs refer to showing the Railcard "on your journey": I wouldn't consider being on the platform at your origin station to being "on your journey" as no travel has taken place.
From memory the situation regarding platform access is a bit confused, but it's certainly not necessary to have a valid travel ticket. Showing a gateline assistant a ticket which is
not valid though is not a good idea!
Practically speaking though, if there are automatic barriers the ticket will almost certainly open them without a problem. If the friend aims to be on the platform a few minutes early she can always ring the OP to ask them to get off and show the Railcard.