Update values using AJAX in field widget form
I generally do something like this:

function gft_enable_display_callback($form, $form_state) {
  // Get the array parents of the triggering element
  $parents = $form_state['triggering_element']['#array_parents']; 
  // Pop off the triggering element as we usualy want the one above it
  array_pop($parents);
  // Get the element from the nested array
  $element = drupal_array_get_nested_value($form, $parents);

  return $element; // or we can return $element['someotherfield']
}


We can also use the drupal_array_get_nested_value to retrieve values from form_state['values'] as needed.