IPhones sharing neither Apple ID nor iCloud somehow still sharing data My girlfriend has recently acquired her sisters old iPhone 5s running latest iOS version as of writing.

Her mother has an IPhone 5s as well, under similar conditions.

Both devices are linked to distinct iCloud and Apple IDs.

My girlfriend's phone contains a number of contacts belonging to her mother. When she deletes one, it is removed from her mother's phone as well.

How do we prevent this?
This happened to me I gave my husband my old apple 5s but didn't wipe it clean first. He deleted 3-400 of my contacts in a 1 hour period since they were still using the same iCloud account. From experience I now know to rest the old phone to factory default and set it up that users own Apple ID. Problem solved except couldn't recover my contacts since a new update was being performed and didn't have a current backup.