More formally, you might consider drawing to their attention any elements of the Code of Practice on retail information for rail tickets and services that you think might not have been followed. There may be computer logs showing what information was (or was not) searched for and displayed and CCTV might have picked up the episode.
But it might just come down to one person's word against another's.
not to give any information which it knows to be inaccurate or misleading, or any opinion not based on verifiable fact
Retailers should take steps to ensure that the information they provide is, to the best of their ability, accurate and truthful and that the way in which it is presented does not deceive passengers or lead the passenger to take a different understanding than that which is actually correct
But it might just come down to one person's word against another's.