You can use splenda. Use an Angel Food cake resipe using spenda instead of sugar. Then, take 1- 8 oz. package of cream cheese and 1- 8 oz. of cool whip. Let the cream cheese come to room temperature and cream it, add sugar substituet, and the fold in the cool whip. Chill in the refrigerator. Spread evenly over the cake. Then you can take any fresh fruit to spread over the top. If you can get sugar free pie filling, strawberry, cherry, or blueberry it is quite tasty.
Tell Mom Happy Birthday and hope she enjoys.