Set post featured image to author image

If you use the save_post hook, every time content is published or updated, the featured image will be refreshed. Note: make sure ACF is set to return an image ID, which is not its default.

add_action( 'save_post', 'wpse_set_featured_image' );
function wpse_set_featured_image($post_id) {
    // get author id
    $author_id = get_the_author_id($post_id);
    // get author's image
    $author_image_id = get_user_meta($author_id, 'your_acf_img_var_name', true);
    // fallback image
    if(empty($author_image_id)) {
        // set to an existing image ID to use as a fallback
        $author_image_id = '4';
    }
    // at last, set the post featured image
    set_post_thumbnail($post_id, $author_image_id);
}