Is there a way to override require_once of the main theme on the child theme?

No, the parent themes functions.php is always loaded, and require_once is not a WordPress API but a part of the PHP language itself

The only way to change it would be to modify the file

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)