Could the guard not have called the station, asked to speak to the team leader or manager there to confirm the customer had been given authority to travel earlier?
This could have been avoided if a system exists to demonstrate authority has been permitted with the passengers name, person giving authority and a brief explanation why
Certainly doesn't excuse the derisory comments made on FB but that can be addressed separately
I'm not sure how differently it works on National Rail, but at Metro our way of working is to give verbal permission. If you encounter an issue further along you can give the location and the member of staffs ID number and they can get in touch with the staff member or, if not, we give the benefit of the doubt and let you on your way. In some cases we'll send an email out to all staff, but in most situations verbal permission is fine because you won't encounter any more staff on your trip. We don't use advance tickets so most of the time we are giving permission for people with incorrect ticketing zones, ticket machine faults, or just bloody good sob stories.
For the guy to go onto social media and make a post about this complaining is rotten in my view. Complaining about your employer is a big enough no-no (we've had a staff member given disciplinary action for complaining about work conditions etc. on Facebook on his supposedly 'private' account (they were friends with a manager who was obligated to report it)), but to complain about passengers is just beyond belief and in my view a sackable offence. We can all have hard days, but the majority don't then go out looking for some Facebook likes/attention.
I'm aware of who the guard involved is, as he was banned from our network on a few occasions and he was also dismissed from London Overground so I'm going to dive back out and just follow along again.