Muddle 8 to 10 leaves of cilantro along with the chile, lime juice and simple syrup in a shaker. Add the grapefruit juice, tequila and about 8-10 ice cubes and shake well. Muddle a few more leaves of cilantro in a 10-ounce high ball glass and strain the contents of the shaker into the glass. Add fresh ice, top with tonic water and stir a few times. Your refreshing drink is ready.