Safest way you can use Bitcoin core wallet is to install it on computer that is not connected to internet,
and as extra security measure, you should create strong login password and encrypt drive,
but most important thing is NOT to keep private key in digital format on that computer, rather use paper/metal backup.
A wallet file does contain private keys.
You can not use a wallet without the private keys.
One can either choose to not store them on a computer at all, which means you need to import them at some point to access the coins, or you store them on a computer.
Since the computer would be offline anyway, it doesn't really matter whether you store them on it, or not. You don't gain much (if anything at all) security-wise when not storing them on your fully offline pc.
I meant that he should NOT keep private key backuo in .txt file or any other digital file format.
People tend to to this sometimes, instead of writing seed words on paper.