Add custom column to Users admin panel with Types user custom fields?

To fix the error, you can do something like this:

 function new_modify_user_table_row( $val, $column_name, $user_id ) {
    $user = get_userdata( $user_id );
    switch ($column_name) {
        case 'les-non-specialistes' :
            return get_the_author_meta( 'les-non-specialistes', $user_id );
            break;
        case 'specialistes' :
            return '';
            break;
        default:
    }
    return $val; //<-- Changed
  }

Regarding the date, which date are you looking for? Which column should it go into (a new one)?

I’ll take a guess – I hope this gets you started. In general, say if you want to add the last modified date, you can do something like this (untested code):

case 'last-modified': //<-- new column that maybe you added above
  global $post;
  $ugly_date = $post->post_modified;
  $pretty_date = date("Y, M, jS",strtotime($ugly_date)); //<-- format as desired.  There are smarter ways of doing this part
  return $pretty_date;
  break;

(reference: http://codex.wordpress.org/Function_Reference/$post)

Good luck,