The Mac failure comes from this piece of code:

            if sys.platform == 'darwin':
                path = self.result[1]
                if path.startswith(('www', 'file:', 'http:', https:')):
                    pass
                else:
                    # Mac Safari insists on using the URI form for local files
                    self.result = list(self.result)
                    self.result[1] = "file://" + path

Before I do anything,
is the comment right about needing to add "file://" for Safari?
is Safari still the default and/or correct way to open a local file?

The code that opens the supplementary help file or url is elsewhere, but it could be changed if outdated on the Mac.