Q: whatsapp, chat transfer

You would have to ask WhatsApp that question.

Here's the heavyweight answer. App developers have a choice of where they store their data. It can be on the phone itself, on their own servers (many do this), or in iCloud. If they store data in iCloud it is in a proprietary format that cannot be read or copied by any other app or by you. If WhatsApp stores its data on the WhatsApp servers it will appear on her phone regardless of the state of her iCloud account as long as the phone number is the same, as WhatsApp is tied to the phone number; that is, it's independent of iCloud. If WhatsApp stores its data in iCloud then there is no way to copy it, as iCloud is encrypted to protect the privacy of the user.

It's also possible that WhatsApp data is accessible using a 3rd party tool such as TouchCopy. This may be able to download the WhatsApp history from the phone, and upload it to a different phone. However, as one of the key features of WhatsApp is its data security (all conversations are end to end encrypted) this may not work either.

Which comes back to the fact that only WhatsApp techs knows how it stores data, which is why asking them is the most useful approach.

Restoring a backup is all or nothing. But if you are moving from one phone to another one this shouldn't be a problem unless you have new data on the new phone that you don't want to overwrite. See: Transfer content from an iPhone, iPad, or iPod touch to a new device - Apple Support

There are PC and Mac applications available that can extract data from an iTunes backup; quite a few of them, in fact (search for "iPhone backup extractor"). The information can be saved in standard file types on the computer, but cannot be restored to the phone.