The majority of people I am thinking of will come and find you before boarding, and ask you if they can travel at a different time to their booked journey. They will usually show you their ticket immediately as they ask the question. Provided the train is not a very busy service, I see absolutely no issue with showing a little discretion. There is nothing to stop the person from simply boarding, sitting at the far end of the train and hoping that you don't find them before they reach their stop, after all. The 'rules' say that we shouldn't excess standard season ticket holders up to first class, and that on a train leaving Paddington in the middle of the evening peak the TM should force everybody quietly standing in the first class vestibules to cram themselves into the back end of standard class even though they're only going to Reading, and that we should charge anybody who has a railcard that's one day out of date, and so on. But most of us don't