How do I retrieve a users’ last 5 posts?

Basically, when you want get posts, you should think about WP_Query. It’s already in the docs for your question.

You can get posts by an author like this

$query = new WP_Query( array(
    'author' => AUTHOR_ID,
    'posts_per_page' => 5
) );

You can even get posts by more than one author, like this

$query = new WP_Query( array(
    'author__in' => array( AUTHOR_ID, ANOTHER_AUTHOR_ID ), 
    'posts_per_page' => 5
) );

Because WP_Query is sort by date and order descending by default so you don’t need specify that information.