Username Accounts cannot be deleted, and if your friend's blog is a Private blog you will want to maintain the username account as you will have to log in to WordPress.com to read it.
Apparently when you signed up for a username account you opted to include a blog, but you can delete it following these instructions. http://en.support.wordpress.com/delete-site/ Please note that once deleted, no one can use that address again, even you. If you think you might want to blog at that site in the future, you can make the site Private rather than deleting it by following these instructions. http://en.support.wordpress.com/settings/privacy-settings/
If you choose to delete your site, you can simply walk away from the username account. Before doing that, here are some excellent suggestions from timethief, a fellow forum volunteer http://en.forums.wordpress.com/topic/cancellation-7?replies=2#post-1063855