HELP! Lost all conversations

Hi, my friend has my old phone, but we installed already whatsapp for him on our apple id. So, the appstore asked him to update whatsapp and he needed our password for that, but he deleted the app and redownloaded it with his own apple id. Now, he lost all of his whatsapp conversations and we dont know how to get it back. Someone please help me! I would appreciate it a lot! (: