Hi.This is how he/she could have separated the mixture.
Instead of mixing with water, he/she could have dissolved the mixture with 100% alcohol which dissolves the sugar and leaves salt,filter sugar and alcohol solution to obtain salt then evaporate the solution to remain with sugar.
I hope this was helpful.I just thought to add it though it was not in the question.
Oct 22nd, 2015
because salt and sugar did not react.The substances remained in their form but the problem was the solvent he/she used to dissolve the mixture.