Redirect specific author posts to another url

First of all… [is_author][1] doesn’t check if given user is author of current post. Straight from Codex:

This Conditional Tag checks if an Author archive page is being
displayed. This is a boolean function, meaning it returns either TRUE
or FALSE.

So it won’t help you.

Another problem with your code is that you perform this check to early. If this code is pasted in functions.php, then the if condition will be checked before query is parsed.

So how should it really look like? Something like this:

function wpse_redirect_posts_to_new_page() {
    // if this is not a single post or not of type post, do nothing
    if ( ! is_single() || 'post' != get_post_type() ) {
        return;
    }

    $post = get_queried_object();  // it's single, so this will be a post
    if ( AUTHOR_ID != $post->post_author ) { // change AUTHOR_ID to real ID of the author
        return;
    }

    $url = parse_url(get_the_permalink());
    $newdomain = 'https://www.example.com';
    wp_redirect($newdomain . $url['path']);
    exit;
}
add_action('template_redirect', 'wpse_redirect_posts_to_new_page');