So, to provide an update as promised: the application was successful and the individual now has a Disabled Person's Railcard. For the submission, we uploaded a scan of the specialist's letter giving the diagnosis of multiple sclerosis, along with a short note of explanation along the lines suggested in the MS Society forum linked to above. There was an odd reply a few days later asking to see evidence of her epilepsy medicine (?!), to which we politely replied with the note reattached. Silence for several weeks - and then the download code for the Railcard sent without any further queries.