Treat this like any other professional project:
* create and use separate accounts for hosting provider and domain registrar. Assuming that your friend's business doesn't have an email, create a gmail account that you consistently use when signing up for services.
* Set payments for hosting/domain to automatic renewal.
* Make sure you can easily hand over the project to another developer if the need arises. This is why you create seperate login accounts for the customer, all connected via the same email address.
* Yourself as admin? Well, someone has to do maintenance; even the most basic sites require backup, adding content, updating software/libs/frameworks, etc. Just make sure you get compensated for the work you do.
* I would recommend using your own Wordpress installation because it's cheap, robust and easy to grow with. Pretty much any dev out there know their way around WP. I would NOT develop a solution from scratch nor would I use Tumblr or Posterous (way to limiting). Wordpress.com? Not worth it it the long run: even basic features cost and you're not in total control of your data (warning: if there won't be anyone doing maintenance, then go for a hosted solution! You do NOT want to leave a WP installation without regular checkups).
* There are countless of free WP themes at wordpress.org. For even more themes, ThemeForest is unbeatable wrt price and quality.