How to get my iPhones to stop syncing with each other?

Ok so my mom used to have an iPhone 3. Now she has an iPhone 4s and that is her everyday phone her old one now is pretty much used as an iPod touch.well I save my friends as contacts in this phone and it goes to hers. She doesn't know I use this so I want them to stop syncing contacts

Https://discussions.apple.com/thread/3927685?start=0&tstart=0 please look at here