I can confidently say that I never got anything for free in my time working in food retail, and certainly never got a benefit from a company in the same industry that I didn’t work for. Expecting a free ride from another company just because you work on trains is no different to helping yourself to free food just because you work in a shop.
The driver concerned would have free travel anyway, likely a Freight Duty Pass or other agreement.
The issue here was not about getting a free ride but being let on an empty train.
As is often quoted by TfL when the Tories go after the nominee passes, there is no cost associated with it as they don't need to put on any extra services to cope with the demand of staff pass users. (Please don't start calculating minute percentages of diesel or electricity).
Now in this case, if the guy seemed alright, I'd have cleared it with control and let him on, but politely asked him to sit in the passenger area and not in the cab. Letting control know also means that if the worst happens they can let the emergency services know there is an extra person to look for.
An additional risk of being on an empty train is that it can be diverted and held in all sorts of places. The reason I'd consider staff and not passengers as in the worst case the driver in question will have PTS and can get off the train where passengers or staff without PTS would be stuck.
I'll be the first to admit that I get really irritated after the end of a long day when there is service disruption that delays me getting home. I won't take to social media about it though, now that I live 10 minutes from my crew depot I just jump in an Uber and open a beer when I get home.
Lastly, I'll add that I used to run tours that used vehicles. I often had friends ask me to join for free. My reply was that if the evening before there was an available seat then we will work out the cost for the permits that were required. However, if I was giving out free seats at other times I'm losing out on the £100 that a paying customer could have given me for the same seat. When you worked in fast food if you took a burger the company is losing out on the £4ish someone would otherwise have paid for said burger. There is no such costs associated with staff travel, this is the difference in your scenario. However, again, this issue was not about free travel.