Important: We strongly advise against signing into other people’s iCloud accounts on your device.
PvZ 2 syncs your profile and game state to your iCloud account. If someone logs you out of your iCloud account on your device, then logs in with their iCloud account, the game will sync from your device to their iCloud account, granting that person permanent access to your profile information. This could result in their deleting your entire game!
You can log into different GameCenter and Facebook accounts on your device with no risk to your data integrity or security. It is only the iCloud login within the Settings menu that will cause these issues.