My wife's chats with her deceased mother are gone. Need help restoring them.

Trying to restore chats with my wife's deceast mother
Hi everyone.

My wife's mother died 3 years ago from cancer and besides a few pictures on her phone, her most important way of remembering moments with her was looking at her latest chats in WhatsApp.

however, even though just last month everything was fine, today she wanted to look again at her chats in whatapp but couldnt find any chat with her at all and couldnt find her as a contact in whatapp either.
whatapp says that she needs to make sure the contact isnt hidden or something. not her whatsapp contact. her cell phone contact.
In whatsapp there is an option to unhide hidden contacts (its not in english so maybe im translating it wrong) but the V is already ticked, yet her mother isnt showing.
When we look at her mom's contact, android tells us that she also has a Viber account but doesnt show the Whatapp account. it disappeared. one of my wife's friends can still see my wifes mom in whatsapp, but my wife cant and whatapp says this contact might be hidden or blocked or something in the contacts settings or something of that sort.
I couldnt find any option to hide or unhide contacts in my wife's Xperia Z5 conpact phone so im asking here -
Is there such an option? if there is - where? how to access it? how to make sure all contacts are visible to all Apps?!
If its a Whatapp issue - how can this be resolved?
We just want to see the old chats again. obviously we dont need to start any chats since she's gone.

Please, help. It breaks my heart seeing my wife unable to re-read her last words with her mom. She misses her and looking at these chats helps a bit.

Thanks.

her mother's number is indeed in the contacts. of course.