A few years ago I made a very stupid mistake on a London bus. I was going into work early during a period when everything was pretty grim there and I had a lot going on in my head, not sleeping too well etc etc - yes, this is an excuse, but just for context. I got onto the bus with my Oyster card in my hand, touched in (as I thought) and sat down.
As I sat down, I noticed that standing at the front near the driver there was an inspector, so I didn't put my Oyster card away. He didn't start inspecting tickets, which I thought was odd, but I didn't think much of it as I was reading and trying not to think about the day ahead. Eventually he came up the bus to me and asked to see my Oyster card, which of course I immediately gave him. He checked it and said 'You didn't touch in'. I was astounded. I said immediately that it was an honest mistake, as I always touched in and out, as the card would show.
He said (and I am absolutely certain this was a lie) that both he and the driver had spoken to me as I got on and pointed out that I hadn't touched in. He had given me a few minutes to put things right by coming back and touching in. I was flabbergasted by this. If he'd spoken to me when I got on I would immediately have realised my mistake and put it right. I didn't have headphones on and would have heard.
He then took the card and found (I think to his disappointment) that there was plenty of credit on it, I had indeed been touching in and out every day and I had no record of fare dodging. He issued me with a penalty notice and I had to pay some huge sum (later I think, not on the spot).
It was excruciatingly embarrassing and I wish now I'd complained/appealed. However, at the time I decided that it would just increase the unpleasantness of that whole period and paid up through gritted teeth. I've been absolutely paranoid ever since about checking that I've touched in on the bus, as I'm convinced that if it ever happened again they would prosecute.
Sympathies to your DD, OP. Probably good advice to film encounters like that.