How would I correctly implement a new if statement in a child functions file?

you put a , instead of ; at end of $advert_details. you will better see this kind of error if you better indent the code.

add_action('wp_enqueue_scripts', 'theme_enqueue_styles', 12);

function theme_enqueue_styles() {
    wp_enqueue_style('parent-style', get_template_directory_uri() . '/style.css');
}

if (!function_exists('adifier_custom_meta')) {

    function adifier_custom_meta() {

        $meta_boxes = array();

        $advert_details = array(
            array(
                'id'                => 'advert_cond',
                'name'              => esc_html__( 'Condition', 'adifier' ),
                'type'              => 'select',
                'options'           => array(
                    '0' => esc_html__( '-Select-', 'adifier' ),
                    '1' => esc_html__( 'New with tags', 'adifier' ),
                    '2' => esc_html__( 'Manufacturer Refurbished', 'adifier' ),
                    '3' => esc_html__( 'Used with tags', 'adifier' ),
                    '4' => esc_html__( 'For Parts Or Not Working', 'adifier' ),
                ),
                'values_callback'   => 'adifier_get_advert_meta',
                'save_callback'     => 'adifier_save_advert_meta',          
            ),
        );

        return $meta_boxes;

    }

    add_filter('cmb_meta_boxes', 'adifier_custom_meta');

}

you can also try a editor with syntax analyse like Eclipse PHP :
https://www.eclipse.org/pdt/