All you really need to do is append /feed/ to the end of get_author_posts_url(), e.g.:
<?php
$author_feed_link = get_author_posts_url( get_the_author() ) . '/feed/';
?>
<a href="https://wordpress.stackexchange.com/questions/23177/<?php echo $author_feed_link; ?>">Author RSS Feed</a>
Codex ref: get_author_posts_url(), get_the_author()
Note that get_the_author() must be used within the Loop; otherwise, you’ll have to have some other means of getting the Author’s user ID.