Well, on the one hand, I can see why this is a problem from the teacher's perspective; after all, a child who is talking to others during 'quiet' seat-work time for the class is interfering with other students' learning.
But it does seem a bit much to place all of the responsibility for this on the shoulders of a young child. After all, he is FINISHED with his work, and has demonstrated clear mastery. (In case you might think I'm not sympathetic here, I was this kid... the "talker" who was done-done-done before everyone else; so I understand completely what your DS is up against in terms of his own nature.)
Obviously there is something the teacher is completely missing here, because the teacher SHOULD be giving your DS something to do which meets her need for classroom control and HIS need to be engaged while in that classroom.
As a stop-gap measure, can you brainstorm some "quiet" ways for him to spend time that won't distract his classmates? Maybe by asking him why he thinks this other classmate doesn't seem to "get into trouble" with the teacher, even though she finishes early as well?
Then present that list to the teacher? Maybe if you presented this as "ideas to improve {DS} behavior and have him learn to support his classmates' learning" or some such thing? (Yes, I know, it's sucking up... a little... but if it improves things, is that such a horrible thing?)
