Oh dear, your friend made a classic error. Please read this > WordPress.com vs. WordPress.org: The Differences > http://support.wordpress.com/com-vs-org/
You have a free hosted WordPress.com blog linked to your username. If that's the blog in question then you will have to get your friend to get a refund for the third-party theme purchased because we cannot install any third-pary themes > http://en.support.wordpress.com/themes/adding-new-themes/
The only themes we can use on free hosted WordPress.com blogs are those found here > Appearance > Themes
If you find that is a deal breaker for you then you can hire a web host and set up your own self hosted WordPress.org install from http://wordpress.org/