Why did you give your password to that guy who is now using your Yahoo ID and changed the whole set up. If he is not a very close friend he would have changed the username and password. You were not supposed to expose your Password to anyone even if he is your close friend. The best thing is to forget about that ID and Password and create a new account for yahoo. The only team whom you can contact is the Yahoo Team but your so called friend is the owner of your Yahoo ID now.