i gave my old iPhone 3gs to my friend. It had my contacts on it and when he erased some contacts it erased them on my phone too.

I gave my old iPhone 3GS to a friend after re-setting phone to the factory defaults. But obviously that didn't get rid of all my contacts, and when he erased them, they've been erased from my phone too. I have mine backed up in iTunes, so I can restore them, but how do I stop it happening again? Thanks for any help!