Bring sugar and water to a boil, stirring to dissolve sugar, remove from heat and allow to cool completely.
Skin peaches by immersing in boiling water for 1 minute, then remove and cool before slipping off skins. Slice peaches and place in food processor or blender with cooled sugar syrup and blend until liquefied. Strain liquid and stir in lemon juice.
Immediately before adding to ice cream maker, stir in chilled Champagne and egg white and freeze following manufacturer’s instructions. Spoon sorbet into a non-reactive container and freeze for at least 2 hours before serving.