Whenever a friend gives up a phone number, you should make sure to delete the number from your phone's address book. As it is common practice for mobile providers to recycle numbers, you may incorrectly identify an account in WhatsApp as your friend's account, when in fact the account belongs to the new phone number's owner.
WhatsApp exclusively uses phone numbers to identify accounts and we display the names you have saved in your address book for those contacts. The names you see in your WhatsApp Favorites list (Contacts tab on Android), in a specific chat and in the Contact Info screen inside WhatsApp are not created within the application.
WhatsApp Support Team