Your nutritionist is talking about the sphincter of Oddi which a muscular valve controlling the flow of bile and pancreatic fluid into the duodenum. It is the path that the Gallstones must pass through. It is a restriction in the biliary path and the place in which stones get held up but they don't stay stuck there for ever. They are just as likely to be impeded here if you are not flushing that is what commonly cause gallstone pain. When the gut enters is "housekeeping" cycle the stones pass.