Just contact YouTube and say it was hacked. You'll need all the information for the account but I'm sure your friend knows all of that.

GO to You must login or register to view this content., scroll all the way to the bottom were it says help, click help and it will take you to a page that has different tabs of what you need help with ... find Create & Manage Account, click account information, you will see four tabs go to the second one down and click "I forgot my Password or Username, How do i reset my password?" It will take you to a page with help and instructions and if that don't help, scroll to the bottom were it says contact us and send them an email about your problem. Hope any of this information helped you out.