author_tags.php

<?php
function get_author_tags($id) {

    $user_tags = array();
    $query = new WP_Query(array(
        'posts_per_page' => -1,
        'author' => $id,
    ));

    while ($query->have_posts()) {
        $query->the_post();
        $tags = (array)get_the_tags();

        foreach ($tags as $key => $val) {
            if ($val->term_id > 0) {
                if (!array_key_exists($val->term_id, $user_tags)) {
                    $user_tags[$val->term_id] = $val;
                }
            }
        }
    }

    return $user_tags;
}
?>