I found this code

 function open_external_links_in_new_tab() {
  echo '<script type="text/javascript">';
  echo 'var links = document.getElementsByTagName("a");';
  echo 'for (var i = 0; i < links.length; i++) {';
  echo 'var link = links[i];';
  echo 'if (link.hostname != window.location.hostname) {';
  echo 'link.target = "_blank";';
  echo '}';
  echo '}';
  echo '</script>';
}
add_action( 'wp_footer', 'open_external_links_in_new_tab' ); 

Kindly review

What about this

 function open_external_links_in_new_tab_bp_activity( $query_string, $object ) {
  if ( $object != 'activity' ) {
    return $query_string;
  }
  ?>
  <script type="text/javascript">
  var links = document.getElementsByTagName("a");
  for (var i = 0; i < links.length; i++) {
    var link = links[i];
    if (link.hostname != window.location.hostname) {
      link.target = "_blank";
    }
  }
  </script>
  <?php
  return $query_string;
}
add_filter( 'bp_ajax_querystring', 'open_external_links_in_new_tab_bp_activity', 10, 2 );