Gil Roth interviewed Sorel on Virtual Memories Show 462: Ed Sorel, and the two of them spoke briefly about Richard at minute 53:30 or so.
Sorel said, "There was one guy who worked like I did. We had the same methods of working, but he was every bit as good as I was. He was terrific. Richard Thompson. I only got to meet Richard Thompson once, and I kissed his hand. I thought he was just terrific. His stuff made you smile, just to look at the drawing."
If I recall correctly, Richard also visited Sorel at his studio when he was still a student, or just afterwards. I'm sure Nick Galifianakis can tell us for sure. Sorel is probably recalling the time in 1997 he had an exhibit at the National Portrait Gallery and Richard undoubtedly went to the opening talk. But perhaps they met elsewhere...
Richard mentioned Sorel twice on this blog.