Show tag cloud with tags from a specific user (add in functions.php).

author_post_tag_cloud_tags.php

add_filter('widget_tag_cloud_args','author_post_tag_cloud_tags');
function author_post_tag_cloud_tags($args) {
  
	if (is_author()) {
		global $post;
		
		$wp_query = new WP_Query("showposts=-1&author=".$post->post_author);
		$author_tag_ids = array();
		
		while($wp_query->have_posts()) {
			$wp_query->the_post();
			$post_tag_ids = wp_get_post_tags( $post->ID, array( 'fields' => 'ids' ) );
			$author_tag_ids = array_merge($author_tag_ids, $post_tag_ids);
		}
		
		wp_reset_postdata();
		$args = array('include' => implode(',',$author_tag_ids));
	}
	
	return $args;
}