Set user billing and shipping country programmatically in WooCommerce on your child site.
Snippet Type
Execute on Child Sites
Snippet
$country_code = 'US';
// Get the WC_Customer instance object from user ID
$customer = new WC_Customer( $user_id );
$customer->set_billing_country( $country_code );
$customer->set_shipping_country( $country_code );
$customer->save();