Single meta_query query using OR instead of AND in request’s WHERE statement

I believe the shortest and best approach is to (1) modify the query:

function search_faqs_metadata($query) {
  if(!is_admin() && $query->is_main_query()) {
    if($query->is_search) {
      $query->set('meta_query', array(array(
        'key' => '_faqs',
        'value' => $query->query_vars['s'],
        'compare' => 'LIKE'
      )));
    }
  }
}
add_action('pre_get_posts', 'search_faqs_metadata');

and then (2) replace the AND with OR via the get_meta_sql filter:

function change_meta_key_where_from_and_to_or($sql) {
  if(is_search()) {
    $sql['where'] = substr($sql['where'], 1, 3) == 'AND' ? substr_replace($sql['where'], 'OR', 1, 3) : $sql['where'];
  }
  return $sql;
}
add_filter('get_meta_sql', 'change_meta_key_where_from_and_to_or');