Ask him if the wallet is not "watch-only" address, if it is, then the problem is obvious.
If the wallet is "watch-only" which it is not actually, shouldn’t he be able to watch the balance as the amount is visible in the wallet? Watch only wallets are supposed to be check the balance only I guess, isn’t it?
Normally, yes. But not if blockchain.com has completely nuked the feature.
I used to have a couple of watch only addresses in blockchain.com I used for testing but months later (or more than a year lol) they all disappeared. I noticed this quick cause my balance reverted to $0. Tried searching for a way to import watch-only addresses but nothing...
They've also omitted the watch only feature in their addresses guide
January 22, 2020 snapshot
To import a bitcoin address from a third party platform, click on + Import Bitcoin Address and select Existing address generated outside this wallet. Enter the bitcoin address or the private key. If you import a bitcoin address, it will be imported as watch only (non-spendable) and you will not be able to use its funds. Next, select an existing sub-wallet to transfer the funds to, or leave this blank if you want the funds to remain in Imported Addresses. Click Import to complete the process.
Updated version (September 20, 2021)
To import a bitcoin address from a third party platform, click on + Import Bitcoin Address and select Existing address generated outside this wallet. Enter the private key of the bitcoin address you want to import. [should've been here] Next, add a label for the address (optional) and select an existing sub-wallet to transfer the funds to, or leave this blank if you want the funds to remain in Imported Addresses. Click Import to complete the process.
But don't worry we can only start considering your friend fell for a watch-only scam once it's confirmed the address/es where funds are supposed to be can't be found in their blockchain.com wallet (HD wallet/imported/archived). Best to narrow down the possibilities one at a time