Who really gives a stuff if a guard lets another guard sit on the comfy cushions for free? Certainly not me. I wish I had that perk, but I don't because I work for a University, so there you go.
But technically giving a product for free when you should charge is depriving your employer of income. That's theft from your employer. If you're caught doing it in a bar or in Tesco you'll be sacked, and we've established that first class is a premium product that is different from the standard product.
That's a matter for the employer and employee though, and I can't imagine management giving two hoots unless they really don't like the member of staff involved. Especially given how temperamental those pesky Avantix machines can be.
I don't agree it's depriving your employer from money. If I get on a train with a staff pass I will sit in standard for free. If the guard invites me into first I will sit in there but wouldn't have paid the premium in the first places as I'm only in there due to the invite. Therefore the TOC was never going to get an upgrade from me, just an empty first class seat. So me sitting in there is in no way depriving them of income. I would be travelling free on that train weather I sit in standard or first.