Displaying user data within ACF relational field (user type)

You don’t actually need to call get_userdata(), all the data you need should be in the relational field array:

EDIT: Add post ID and User ID to calls to get_field().

<?php
$posts = get_sub_field('who_to_show');

    if( $posts ): ?>

        <?php foreach( $posts as $post):

            setup_postdata($post);
            $user_info = get_field( 'relational_user_field', $post->ID );
            $image = get_the_field('author_image', 'user_'.$user_info->ID );
            $role = get_the_field('role', 'user_'.$user_info->ID );
            ?>

            <h3><?php echo $user_info->user_nicename; ?></h3>
            <img src="https://wordpress.stackexchange.com/questions/312929/<?php echo $image["url']; ?>" alt="https://wordpress.stackexchange.com/questions/312929/<?php echo $image["alt']; ?>" />

        <?php endforeach;
    endif;
    wp_reset_postdata();

endif; ?>

Obviously swap 'relational_user_field' for your actual field name.
The data returned in the user array is the same as the return values in the_author_meta