which city is Mrs.John leaving for ----{看} her son tomorrow.

to do形式的用法之一：表示应为动作的目的.另外,有时间副词Tomorrow,用to do,表示将来发生.
先行词是which city.
谓语为is.
Mrs.John leaving for 修饰which city.
to wisit her son tomorrow tomorrow是补充成分,表示目的.