Combine rhubarb and water in a medium saucepan. Bring to a boil; cover, reduce heat, and simmer 10 minutes. Strain rhubarb through a sieve into a bowl; discard solids. Add sugar substitute to rhubarb liquid; stir well to dissolve. Cover and chill at least 1 hour.
Spoon 1/2 cup ice cream into each of 2 glasses. Pour 1/2 cup rhubarb liquid over ice cream in each glass.