Mix the fish sauce, lime juice and brown sugar in a small bowl; set aside.
Lightly toast the coconut in a skillet over medium heat; cool. With a sharp knife, peel and segment the grapefruit, removing the white pith. Toss the segments in a bowl with the coconut, peanuts, mint, chili sauce and scallions.
Heat 1 tablespoon oil in a large skillet over medium heat. Add the pork and cook, breaking it up, 4 to 5 minutes (do not brown). Transfer the meat to a bowl and drain any excess liquid; set aside. Raise the heat to high, add the remaining 1 tablespoon oil to the skillet, then the ginger and red onion; cook to soften, about 1 minute. Add the onion to the meat, pour in the fish-sauce mixture and gently toss.
Divide the pork mixture among lettuce leaves and top each with some of the grapefruit salad.