Is theory right or wrong? Not the way to look at it.
Music does not have a 'way'. Music is by its very nature, subjective. John Lydon (Rotton) can hate ELP but ELP sold lots of records. People liked their rather ponderous approach. I'm guessing the ELP lads didn't much dig the Pistols.
Short of it being, music isn't a path of right or wrong but a path that is right for you. I was fully convinced that I did not need theory and that I had a good enough ear. I had people that reinforced this idea by being very complimentary of my playing at the time.
I stopped playing for a good few years to start a business and get 'life' going. Well, life is going and a few years ago, I decided to start playing again. I found that I played the same old stale stuff I always did. So, I started digging in to theory and learning some very cool things that have expanded my playing.
A creative player will never be limited by theory but will include that in their playing arsenal. Your friend is creative and that is a blessing. Theory has nothing to do with that and maybe your friend could even be better than he is with this knowledge.
You only limit yourself by how you think and not by what you know.
