When is the best time to prune?

my son in laws stepfather has bin and cut back badly 2 lilac trees and all the shrubs he has in his garden will they flower next year and is this the right time to be cutting anything back,he has been trying to do this all year,im sure his timing is wrong and i know it will have killed some of the shrubs,my granddaughter is so upset it was done while her father nipped out i want to tell her we will get them back,does anyone know if the lilac trees will flower next year thank you