Create a new account (for your friend maybe) and give it admin privileges. Then login as this new admin user, and delete the old admin user (then delete their files - after backing up of course).
You need to be logged in as an admin to delete an account, and of course, you cannot delete the account as which you are logged in.
This is the easy way.
You can actually set up you computer to have no users at all, and start the welcome routing to set up the computer, but that's a little more complicated. The instructions are in a thread on this forum... do a search, if you're interested.