For the lavender simple syrup: Add the sugar, 1 cup water and the lavender to a saucepan and bring to a simmer over medium heat. Simmer until the sugar is dissolved; remove from the heat. Let the mixture sit overnight and infuse, and then strain into a jar. Store in the refrigerator for up to 1 week and try not to drink it alone.
For the cocktail: In a tall glass filled with ice, add the tequila, grapefruit juice, lime juice, 1/2 ounce lavender simple syrup and salt and stir. Top with the light beer, stir lightly and garnish with a sprig of lavender.