This is an old comment but I'll interject because I'm heading to music again after... 20 years?
I was in music school, guitar. Never had I touched one before the first class. They had students there for years in the same class (it was a mix with a teacher that would jump from each one to the other and be more personalised). In the second class ever, the teacher did the same exercise for all - he untuned all the students' guitars and gave them back for us to tune them "by ear". There was a score point (0-20). Students that've been there for longer got 18...19... newer ones around 10, 11... when it was my turn I felt so ashamed - it was going to be terrible in my only 2nd class ever!! Well, I got a perfect 20, no one had ever achieved that. I didn't like the looks I received though... He said I had a musical ear and that it was quite the gift and helped him understand how I learn things.
I never learned how to read music (just very basic stuff) but I could create my own songs and repeat what I heard just fine. And I understood basic theory from that - my mind worked in making sense out of the practice, hearing, being aware and having a good ear. It was the opposite of what is normal I think... not sure because never went through the "normal" music learning process.
I completely forgot about reading music and did it "my way". As someone who fights with focusing, I drew my own diagrams to understand what I was doing. And the music teacher was just fine with that - "as long as you understand the very basics of a sheet, do it the way your brain works best with". So he adapted my learning to playing by ear along with some challenges to keep my memory fresh of the basics of music reading. It was interesting being in a class where you had kids 100% needing the hard theory and reading music and others doing something very different.
Long story short, I'm trying to relearn music (got out of it for too long) and went into the theory and it really... does not work for me. I still am drawing things my own way, have visual tools around and I'm slowly (painfully so...) starting over. And maybe realising reading music just isn't my way/is not the best way for my brain to understand/learn. Now with guitar + piano. The musical ear is coming back, slowly I think - I completely lost it because I played nothing for so long. Except it's way harder to learn later in life IMO and as a kid you don't have a million other worries.
So... I say - music theory books for children are great, some YouTube videos that give an excellent overview of what music theory is without diving into much detail make you aware of the basics and then you play and figure it out as you practice. There are multiple free PDF's of books/essays with many diagrams to help explain it visually, a sort of "Music theory for dummies" or "101" thing.
It is possible. You will learn a bit of notation inevitably IMO but not to the deep end, just enough to help you make sense of the practice. Glossaries on many websites help too.
Good luck to your friend, I hope I can go back and tune my ear again!