Get meta value and associated user meta

Just alter your database query to also grab the user ID, and then return an array of data objects (as opposed to just the meta value):

function get_meta_values( $key, $type="workout", $status="publish" ) {
    global $wpdb;

    if ( empty( $key ) )
        return;

    $r = $wpdb->get_results(
        $wpdb->prepare( "
            SELECT pm.meta_value, p.post_author FROM {$wpdb->postmeta} pm
            LEFT JOIN {$wpdb->posts} p ON p.ID = pm.post_id
            WHERE pm.meta_key = '%s' 
            AND p.post_status="%s" 
            AND p.post_type="%s"",
            $key,
            $status,
            $type
        )
    );

    if ( $r ) {
        $user_ids = wp_list_pluck( $r, 'post_author' );
        cache_users( $user_ids );
    }

    return $r;
}

$workout_leader = get_meta_values( '1_rep', 'workout' );
foreach ( $workout_leader as $data ) {
    echo $data->meta_value;
    echo $data->post_author; // User ID

    // Any other user-related data.
    echo get_the_author_meta( 'display_name', $data->post_author );
}