Is this will work?
I never heard that the 12 words seed from blockchain when you import it to Electrum you will see your funds?

So I think this will not work so the only solution is to report this to the support and send all of his funds to the new wallet in Electrum.

Just use a much better wallet like Electrum and avoid Blockchain wallet. The issues with synching and showing wrong wallet balances confuses newbies, and they end up losing Bitcoins

First check the available address in your blockchain wallet using a bitcoin explorer like blockchair.com to be sure if it really has the bitcoins. If it does;

1. Export your Bitcoin address from Blockchain wallet, and Import it to Electrum. Head over to your blockchain wallet and select backup funds.

2. Copy your backup phrase, write it down and keep it in a very safe place.

3. Download Electrum from the Official site (https://electrum.org) and verify the GPG signatures depending on your Operating System before installing it just to be sure you installed the right wallet.

4. Once you have installed it, Open Electrum, Select New and then enter your desired account name and click next to create.

5. Select Standard wallet and among the options select "I already have a seed"

6. Enter your 12 word seed and under the seed options button check BIP39 and click on next.

7.  I believe your script type on blockchain wallet was legacy as the default, so you select Legacy (p2pkh) and the derivation Path stays as m/44'/0'/0'

8. Enter your desired wallet password, click next to create the wallet. You should be able to see your bitcoins and spend them. If are not able to see your bitcoins, check the addresses in the wallet to see if one of them was your default address in the blockchain wallet.

Please keep us updated if you are successful or not