Don't forget to add custom post types in the $post_type = array after the 'post'. Separate post types by comma, make sure you use the post type name not the post type titles. Post type names don't have upper case chars and spaces:

add_filter('get_usernumposts', 'wpforo_user_custom_post_type_count',10, 4);
function wpforo_user_custom_post_type_count( $count, $userid, $post_type, $public_only ){

   $post_type = ; //add comma separated custom post types

   if( function_exists('WPF') ){
      global $wpdb;
      if(is_wpforo_page()){
         $where = get_posts_by_author_sql( $post_type, true, $userid, $public_only );
         $custom_count = $wpdb->get_var( "SELECT COUNT(*) FROM $wpdb->posts $where" );
         if( $custom_count ) return $custom_count;
         return $count;
      }
   }
}

Is it possible that in user activity the custom post types are added to the total blog articles written by a user?

Thanks