client_side_validations.rb

# config/initializers/client_side_validations.rb

...
module SimpleForm
  class FormBuilder
    def self.client_side_form_settings(options, form_helper)
      {type: 'SimpleForm::FormBuilder'}
    end
  end
end

rails.validations.simple_form.js

// app/assets/javascripts/rails.validations.simple_form.js

/*
  Client Side Validations - SimpleForm - v1.5.0.beta.3
  https://github.com/dockyard/client_side_validations-simple_form

  Copyright (c) 2012 DockYard, LLC
  Licensed under the MIT license
  http://www.opensource.org/licenses/mit-license.php
*/

(function() {

  ClientSideValidations.formBuilders['SimpleForm::FormBuilder'] = {
    add: function(element, settings, message) {
      var errorElement, wrapper;
      if (element.data('valid') !== false) {
        wrapper = element.closest('.control-group');
        wrapper.addClass('error');
        $('.controls .help-inline', wrapper).remove();
        errorElement = $("<span/>", {
          "class": 'help-inline',
          text: message
        });
        var helpBlock = $('.controls .help-block', wrapper);
        if (helpBlock.size() == 0) {
          return $('.controls', wrapper).append(errorElement);
        } else {
          helpBlock.before(errorElement);
        }
      } else {
        return element.parent().find("span.help-inline").text(message);
      }
    },
    remove: function(element, settings) {
      var errorElement, wrapper;
      wrapper = element.closest('.control-group.error');
      wrapper.removeClass('error');
      errorElement = wrapper.find("span.help-inline");
      return errorElement.remove();
    }
  };

}).call(this);

simple_form.rb

# config/initializers/simple_form.rb

SimpleForm.setup do |config|
  ...
  config.wrappers :bootstrap, :tag => 'div', :class => 'control-group', :error_class => 'error' do |b|
    b.use :html5
    b.use :placeholder
    b.use :label
    b.wrapper :tag => 'div', :class => 'controls' do |ba|
      ba.use :input
      ba.use :error, :wrap_with => { :tag => 'span', :class => 'help-inline' }
      ba.use :hint,  :wrap_with => { :tag => 'p', :class => 'help-block' }
    end
  end
  ...
end