The 20 word seed phrase is not BIP39 mnemonic seed phrase
I confirm this, and iancoleman.io/bip39 website shows only 18 and 21 length options. So you really need to solve this in blockchain.com wallet.
He will need to confirm ownership of the account using an email address and confirmation link. I think, without access to this email, he can't use a connected wallet.
OP, have you tried to access using the android app?
i wrote old email which i don't have access and new recover email address, but It gives me this message "error re-setting up authentication"
Does that email still exist?
If it is
someone@hotmail.com the email account might have been deleted since you don't use for a long time. Maybe you can create a new email with the same username (if available) and receive the blockchain.com email.
This might work.