IPhones sharing neither Apple ID nor iCloud somehow still sharing data
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.