I emailed DS's teacher about the issues he is having with "R" and she wrote right back. She said she had not really noticed anything going on, but did think DS often seemed "distracted."
She said she will move DS and R to different tables the next time she changes the seat assignments. The kids have "table teams" and earn points for prizes, so she doesn't want to switch anyone to a different table right now. But she did move DS so that he wasn't right beside R.
She also switched R to a different reading group so he and DS won't have to work together all the time. And she will work with the whole class on social skills related to the problems DS and R are having and encourage everyone to work/play with different people.
I was very impressed with her responsiveness and quick action.