After my father in law had died, my mother in law kept his iphone which she still has access to. Even after his phone contract expired and his number had been deregistered, she was still able to look at his whatsapp messages on his phone.
But when we logged in recently recently, WhatsApp refused to start claiming the phone's time and date were wrong which they weren't. After updating the WhatsApp app and restarting the iphone, WhatsApp showed the registration dialog.
The icloud folder on his phone shows empty, so I have no access to any backup file either.
On her own phone, all chats with her deceased husband are no longer visible, while on my own (android) phone, I am able to find our chats by searching not for his name but for his old phone number.
I contacted WhatsApp support to help me recover his data, but the answers I got so far only quoted their faq and seemed unrelated to my inquiry.
What else can I try to recover the old chat data?