Help get specific address private key
It isn't clear why your "helper" would have missed 1.3 BTC at a separate address in the same Bitcoin-core wallet. Did you use the "Account" feature that was recently removed from Bitcoin-core?

I would do the following

Prepare a new empty wallet using your choice of hardware wallets or software wallets (1). Make a careful note of the seed-phrase/recovery-phrase or private-keys. Practice restoring this wallet from only that noted secret information.

Download and install the latest Bitcoin-core onto a clean PC and reinstate the old wallet.dat file into it (2), then use the newly found password to open it 

Let it synchronise (may take days or weeks)

Immediately any money is visible, transfer any money into the other wallet.

If no money is visible, try to find out the status, in the wallet, of the address you say holds 1.3 BTC. Note that the command line interface(3) may be helpful in listing addresses the wallet is keeping track of.



References


https://bitcoin.org/en/choose-your-wallet
Import wallet.dat into a new Bitcoin-Qt client
https://en.bitcoin.it/wiki/Original_Bitcoin_client/API_calls_list