Cut grapefruit on half from top to bottom. Using a large spoon carefully scrape out the flesh and most of the white pith, being careful not to puncture the skin. Fill empty grapefruit cup with sorbet, making sure to compact it so there are no air bubbles.
Wrap with plastic wrap and freeze for at least 6 hours.
When ready to serve, remove plastic wrap and using a sharp knife carefully cut the grapefruit into 3 wedges.