How to uninstall zsh
If your friend changed the command that shells are opened with, you can change it back to the default login shell from Terminal's preferences:



If the default login shell was changed, you can change it back to /bin/bash by running chsh -s /bin/bash.

zsh is included with OS X (in /bin/zsh), but even if your friend installed a newer version of zsh, you don't necessarily need to uninstall or remove it.