Combine sugar and cornstarch in a 2-quart saucepan. Stir in grape juice; cook over medium heat, stirring constantly, until thickened and bubbly. Cook 1 minute.
Add a small amount of hot mixture to egg, mixing well; stir egg mixture into remaining hot mixture. Add butter and lemon juice; return to heat. Bring to a boil, stirring constantly; boil gently 1 minute. Cool. Pour into pastry shell; chill thoroughly.
Combine whipping cream and sugar, beating until light and fluffy. Spread on pie; chill.