Did you mean that your friend installed the Preview for Developers App? Installing that app does not automatically enable your phone to receive preview updates... you need to "Enable" it first in the app itself.
You should find the "Preview for Developers" App in the App List. In it, just uncheck "Enable Preview for Developers" (if it is checked). In most cases, you won't need to restart your phone after that. You don't need to uninstall the app afterwards, as long as this is "unchecked" your phone will not receive preview updates.