My iPhone 5 (iOS 6.1.2) synced contacts from my wife's iPhone 5 (iOS 7.0.2) and changed many of them (changed names to contacts with similar numbers). She uses my Apple ID, but contacts transfer from her phone to mine. Does anyone know why does it syncs contacts?
Pardon me for being slightly out of context here, but I'd suggest creating a new Apple ID for her anyway. Apple now has Family Sharing feature, which would allow you to share media and purchases if you need. Though its available in only iOS 8+ (you'll have to upgrade of course) I recently got to upgrade my personal iPhone 4s to iOS 8 and I think its really useful in sorting out such situations with family and kids.
I have switched off contacts sync with iTunes completely. Leaving iCloud to be only way the contacts sync, hence reducing the confusion and ambiguity.