Learned dyslexia? adult predicament from childhood teacher´s incompetence?
I will answer your questions out of order. I hope that is ok.

6 I have dyslexia and other "stuff". I have been diagnosed as such by a psychologist and diagnostician multiple independant times. 

2 Dyslexia is quantified as a DSM 5 Neurodevelopment Disorders subcategory Learning Disorder.


  “difficulties learning and using academic skills.”


Before DSM 5 dyslexia was identified by a difference between verbal and performance IQ which would effectively measure the severity of the dyslexia in comparison with the persons ambition. I'm sure psychiatrist have a new standard test which is able to perform such functions as well.

1 Dyslexics typically learn coping strategies at school. Without foundation of those coping strategies I know reading and writing would be even more difficult for me. I was taught to read and spell with the Scottish Rite Dyslexia method. Its basicly phonics on steroids. It seemed to work very well for myself and the others with learning disabilities though not all were dyslexics. I would recommend finding curriculum based on the framework the Masons provide and that is at least is phonetic. 

3 Motivating an adult is very difficult. You might do some research on Social Influence which is ethically acceptable manipulation like what doctors use. 

4 Absolutely no one knows what causes learning disabilities like dyslexia all major theories point to something biological involving the brain. Finding the source of the problem doesn't help the person. Only treating the symptoms is rational. 

5 No. Your friend comes from a time just before the major push for treatment of learning disabilities in public schools. Now learning differences and disabilities are given coping strategies and other tools (primarily time and technology) to help them perform and live healthy normal lives.