Q: Contacts visible after reset

Hi, I erased all content and settings on my iPhone 4S and also did a hart reset a couple of times.

When I gave the handset to a friend to use, once he restored the phone from his iTunes backup, He can see all my contacts on the phone and his contacts when we selects show contacts going into mail and contact settings !

This is strange as I had deleted all content on my phone before giving it to him..

I would want to know how to ensure my contact list is erased for real and how can he restore form his backup so his contacts are visible but not mine

Please advise