Thing is that dyslexia is much more than just getting letters muddled up. I can only go on the experiences of those who have told me what it's like, but it's more about recognising patterns - a bit like those "oh, I know this but I just can't find the right word" moments we all have.
It's more than reading and writing too. I used to support a blind lad who was also dyslexic. Yes, he showed the classic signs of dyslexia - very badly written prose, lousy spelling and syntax, and sentences that would morph into something completely different from what they were when they started. But his dyslexia also showed in other ways - like a complete inability to time-manage or stay on track with a task.
Someone else I know who's dyslexic has a Doctorate in Literature and lectures at a university. He's a brilliant musician too, but has a self-confessed difficulty in reading music. He's a nightmare to rehearse with. He'll suggest you rehearse a piece, and you say, 'OK, we'll do that next, when we've finished this." Two minutes later he'll forget about what you were rehearsing and launch off into his own stuff.
This is kind of typical of how he is in other areas. Now, if someone could tell me how to get angry with him over this, it would help, because he's so loveable with it that you just can't be. If he was a kid, he'd be so spoilt. :-)