Prepare a hot fire in a charcoal grill or heat a gas grill or broiler. Away from the heat source, lightly coat the grill rack or broiler pan with cooking spray. Position the cooking rack 4 to 6 inches from the heat source.
While the grill is heating up, make the sauce. In a saucepan, combine the strawberries, rhubarb, sugar, water, vanilla and cinnamon. Cook on medium heat until the mixture just starts to boil, about 5 minutes. Remove the saucepan from the heat and set aside.
Place the angel food cake toward the edge of the grill rack where there is less heat or on the broiler pan. Grill or broil until each side turns brown, about 1 to 3 minutes.
Place the angel food cake on individual plates. Top each piece with 1/4 cup of the strawberry-rhubarb sauce and 2 tablespoons of the whipped topping. Serve immediately.