Real time form validation

Hello, I am stuck with a realtime form validation. I obtained
information on how to build real time form validaion on:

http://blog.andyamiri.com/2008/05/ajax-form-validation-for-rails.html

It has worked great.... but... until now I have not been able to
validate a password confirmation field for the form. The problem is
based on my limitation to modify the code to accept two different
field values to validate. I would appreciate it very much if someone
could help me. Thanks in advance.

Here is the code:

  def error_message_for(model, field, options = {})
    options[:success_css_class] ||= 'success'
    options[:error_css_class] ||= 'error'
    options[:hint_css_class] ||= 'hint'
    tag_id = "#{model}_#{field}_validator"
    js = observe_field "#{model}_#{field}", :function =>
      "new Ajax.Request(
        '/usuario/validate?field=#{field}&value=' + value,
        { method: 'get',
          onSuccess: function(transport) {
            element = document.getElementById('#{tag_id}');
            var output = transport.responseText;
            var css_class = '#{options[:error_css_class]}';
            if (output.length == 0) {
              output = '#{options[:success]}';
              css_class = '#{options[:success_css_class]}'
            }
            element.innerHTML = output;
            element.setAttribute('class', css_class);
          }
        }
      );"
    tag = content_tag :span, options[:hint], :id => tag_id, :class =>
options[:hint_css_class]
    return tag + js
  end

This "helper" triggers the observe_field for changes... when the field
is changed it sends the "value" of the "field" to a function called
"validate" which I show next:

def validate
    field = params[:field]
    user = User.new(field => params[:value])
    output = ""
    user.valid?
    if user.errors[field] != nil
      if user.errors[field].class == String
        output = "#{user.errors[field].slice(2,100)}"
      else
        output = "#{user.errors[field].slice(2,100)}"
      end
    end
    render :text => output
end

which then returns the result and gets displayed on the view... I need
to somehow add the "password_confirmation" field to the process so
that I can then validate that it is equal to the password. Thanks
again.