Highlight another nav item

The page_css_class filter lets you modify the classes each menu item gets.

Here we check if we are currently viewing a singular envira post type and the menu item slug is gallery. In that case we add a class to the array of default classes passed to the function.

function wpd_page_css_class( $css_class, $page ){
    if( is_singular( 'envira' ) && 'albums' == $page->post_name ){
        $css_class[] = 'current_page_item';
    }
    return $css_class;
}
add_filter( 'page_css_class', 'wpd_page_css_class', 10, 2 );