How can I toggle required field?
In my case, I actually didn't need server-side validation; I just needed validation to serve as a reminder to fill out an exposed field. Instead, I added field-level validation with required.

$(context).find('.field-name-field-stop-displaying-after').hide();
$(context).find("input[name='field_stop_displaying_after[und][0][value][date]']").prop('required',false);
$(context).find("input[name='field_status[und]']").click(function() {
    var radioSelect = $(this).val();
    if (radioSelect == 0) {
        $(context).find('.field-name-field-stop-displaying-after').hide();
        $(context).find("input[name='field_stop_displaying_after[und][0][value][date]']").prop('required',false);
    } else {
        $(context).find('.field-name-field-stop-displaying-after').show();
        $(context).find("input[name='field_stop_displaying_after[und][0][value][date]']").prop('required',true);
    }
});