Good on you for helping him like that, as someone who used to main Mid/JG, having my friend support me was really helpful because I consistently got the same level of support every game, so it was easier to focus on what my shortcomings were since some games you just get a bad support and can't play the lane to figure anything out. What made it super easy was that he played all enchanters, so he was literally just sitting in my lane doing his thing and saving me when I made mistakes, but otherwise the lane was up to me to figure out.
That being said, maybe see if he'd be willing to practice other ADCs. What helped me learn ADC the most was MF, or any similarly immobile ADC, but MF is really good because it forces you to realize when the enemy is mispositioning to get the most out of her Q, and it forces you to get good at positioning because you don't have an escape. I was previously playing things like Ezreal, Trist and Caitlyn, but their escapes were just a crutch so I'd fuck up a lot. Now I don't play immobile ADCs because I still hate them, but the lessons I learned on them have definitely transferred over and helped me a lot over the years.