You can also look at Puppy Linux.
It's a live distro. It can be run from CD, DVD, Blueray, USB, even can be installed on Hard drive.
When running live It can save back any changes you made or new programs installed in a savefile on drive you choose, even on the same CD,DVD or Blueray you are booting from in multisession mode.
Please take a look at Tahrpup 6.0 CE
, a 32 bit ubuntu based Puppy linux variant, It can install packages from ubuntu repo from it's package manager. It's just 200Mb iso to download.
There is also a debian based DebianDog
puppy. 150Mb iso.
I use Fatdog64-631
the stable one, and I am testing Fatdog64-700 beta2
. It is 64bit puppy made from Linux from scratch. It's 250Mb iso.
I am only general desktop user. I mostly use Vlc, Firefox, Gimp, Libre-office, Libre-CAD etc...But there are many other sofwares available too.
Also being in ram they start and work so fast. My system gets to desktop after I press power ON button in just 13secs, that too not using SSD, Booting from usb2.0 stick.
Just take a look, it won't mess with anything that is preinstalled on your friend's drive, and gives faster experience than other live distros I tried.
I am new to linux. I have win7 and ubuntu 13.10 installed on my machine. But since I found Puppy in Feb this year, I have been using it and never booted in win7 entirely and occasionally boot ubuntu.