First, use a command line program called
shred to wipe everything from your hard drive.
shred will overwrite your personal data with random numbers so that the original data is not recoverable.
To do this, create a Live USB Ubuntu drive that you can use to boot the computer from. If you can make the live USB persistent then installed files like
shred will remain there for later use.
Using "Try Ubuntu" option in the live USB, install
shred by typing in a Terminal
sudo apt-get install shred
man shred to learn about the program.
shred to wipe the computer's disk(s).
Second, Install Ubuntu. If you use the OEM install option, you can set it up with updates and install other software (like Chrome or Skype) without creating an user-ID for your friend. When your friend boots the computer for the dirst time s/he will be prompted for name language timezone etc. the final setup. See How do I pre-install Ubuntu for someone (OEM install)?.
Hope this helps.