Section grapefruit halves; set sections aside, and drain. Squeeze grapefruit halves, reserving 1/2 cup juice. Combine juice and gelatin in a small saucepan; let stand 1 minute. Cook over low heat, stirring constantly, until gelatin dissolves. Add 1/2 cup brown sugar, stirring until sugar dissolves.
Remove from heat. Stir in half-and-half and lemon juice. Chill thoroughly, stirring occasionally, until mixture begins to thicken.
Beat egg whites (at room temperature) until soft peaks form. Gradually add remaining brown sugar, beating until stiff peaks form. Fold into gelatin mixture. Spoon alternate layers of cream mixture and grapefruit sections into eight 6 - ounce parfait glasses. Chill thoroughly.