function override_billing_email( $email, $order ) {

    $order_data = wp_fusion()->integrations->woocommerce->get_customer_data( $order );

    if ( isset( $order_data['additional_participant_email'] ) ) {
        $email = $order_data['additional_participant_email'];
    }

    return $email;

}

add_filter( 'wpf_woocommerce_billing_email', 'override_billing_email', 10, 2 );