Seeing this thread re-activated, I re-read it. I think a useful lesson can be learnt from my experiences with a friend this last year. My friend is in her early 30s and has never been taught to read music, nor an instrument, but wants to do both.
I offered to teach the basics of music theory / reading music. We had a regular one hour once-a-week session for about 6 months. In that period we covered all of the ABRSM Grade1 theory and fair chunks of 2 and 3 (I didn't tell her that). I reckon she'd get a solid pas if she took G1 now.
Two things I focused on were rhythm and note recognition. I would either give her some music, or write something myself, set a tempo and get her to clap the rhythm. She found this hard, but we made decent progress.
I would constantly ask, 'What note is that'. It was a struggle at first and needed a lot of repetition of how it works (treble clef is a stylised letter G and wraps around line 2 etc). After 6 months, we were getting reasonably OK at this in both treble and bass clef, leger lines were still tricky.
Was she fluent at the end of this period? No, but decent progress had been made. She struggles with time (she has two jobs).
Conclusion? I am generally of the opinion that just about anyone can be taught to read music. Some will pick it up very quickly, some won't. You have to be prepared for people having very odd ideas at times (one person I helped out struggled until I sat them at the piano and pointed out how the note names repeat and that going down from A is G, F, E...). I'm not a teacher and I have not studied the relevant pedagogy my limited experience is that you just need lots of repetition and practice - you just have to keep doing it and become self-reliant and accept it may be slow at first.