Also, rereading the OP. At no point does she say the connecting train was delayed, she says the tube was slow and the train stayed at a red light.
She said that the train/tube was at the red light for 10/15 minutes. There's no way that was in the timetable, that's a delay.
Dyslexia would not stop you reading "valid on 11.30 train only", written in the same size font as the rest of the print. If you can read this, you can read a ticket.
Physical card tickets are pretty clear. The various mobile app and PDF tickets on the other hand often have restrictions which are tucked down the bottom somewhere and difficult to read. I come across plenty of people who make the same mistake. Trust me, if I (someone who is paid to check these things) find the ticketing system to be arcane and overcomplicated then god help the general public.
Today I came across a lady who'd been sold a ticket for another operator. I could see that she'd been genuinely unaware and that the ticket office should have been clearer on the validity when selling it. Therefore, I didn't charge her for a new one and just warned her about the restrictions. Policy was that she should buy a new ticket and have the original refunded but as the guard, I am responsible for my train so it's my decision. Again, in the example the OP has given I'd have given the benefit of the doubt.
That doesn't mean that I'm a walkover though. I later came across two girls who were rather inebriated. The first one paid without question. Her (non-identical) twin sister baulked at the price and then tried to claim that she was 15. Had she been sober, I could have believed that she was 15. What sealed it though was that the two of them were telling all and sundry on the platform that their 18th birthday was last week. The first sister (who had paid the adult fare without question) did little to back the second up and after a few 'jobsworth' insults and a lot of moaning the second paid the fare. She was in tears on the phone to someone the next time I passed but I had no sympathy, she was a liar who got caught out by her own indiscretion.
At no point was I aggressive. Assertive, yes but never aggressive. Raised voices and shouting help no one.
The bit that made my day though was that when I went back to the cab after they'd left, a group of ladies commended me on the way I'd stood my ground and remained professional.
If I'm unsure whether or not someone is telling the truth, I give them the benefit of the doubt. I'd have done so in the OP's case.