Time to part ways, I'd say. I don't think any article will change her mind. She is telling you loud and clear that she is no longer capable of meeting his needs. Better to stop now than to wait for things to get ugly. If after three years she still can't fathom the kid's intellectual needs, it's time to cut bait.
My son was in Montessori school for a bit. It worked very well the year he was 3/4. The directress gave him lots of work with the kindergarten students and brought in things like a 4th grade history text for him to read. He used to just happen to be sitting in the book area reading encyclopedias when parents came to tour the school- I found out about that later. My son was being used as a marketing pawn, despite the fact that he was reading long before he entered her school!
The following year he returned as a 4/5 yo kindy student, and we had to pull him after 3 months. It just wasn't working anymore. I have to say, it was incredibly tough to make that decision, but then we were all much happier.
good luck-
