1) Try different combinations of passwords: It's possible that your friend may have made a small typo or mistake when entering his password. He can try different variations of his usual password to see if any of them work.
2) Use a password manager: If your friend uses a password manager, he can check to see if the password for his cryptocurrency wallet is saved there.
Since OP already ask help about brute-forcing password, it's very likely OP's friend already tried those suggestion.
3) Use a seed phrase: Many cryptocurrency wallets use a "seed phrase" or "mnemonic phrase" as a backup way to recover the wallet. This is a series of words that can be used to regenerate the private key and regain access to the wallet.
Bitcoin Knots website and source code doesn't mention anything about mnemonic/seed phrase support.
4) Use a password cracking software: Your friend can use a password cracking software to attempt to recover the password. Keep in mind that this process can take a long time, and there is no guarantee that the password will be recovered.
Already mentioned several times by other member with more detailed information.
If all above failed, Contact the wallet support: If your friend is unable to regain access to his cryptocurrency wallet, he should contact the wallet's support team for assistance. They may be able to help him recover his funds.
There's no thing such as contact support for Bitcoin Knots. And i doubt developer of Bitcoin Knots (Luke Jr.) have spare time to offer assistance when he lost hundred BTCs recently.
The wallet is with Bitcoinknots, How we could know the type of encryption they are using?
As far as I understand it, BTCrecover supports variations of Core like Bitcoin Unlimited, XT, etc, so I think you can give it a shot even if Bitcoin Knots is not explicitly mentioned. CMIIW.
That should be true. Bitcoin Knots, XT, etc. are fork of Bitcoin Core, so BTCrecover should the job unless the developer bother change format/specification of wallet file.