Either your wallet is incorrect, or your wallet is indeed for a different chain. The error is unlikely to be wrong.
Does the Core Client perhaps need the blockchain? Or does it open the old wallet without it?
With or without the blockchain, it'll show that error if the wallet.dat is from another chain or Altcoin.
He can force to load it by starting 'Bitcoin Core' with
--walletcrosschain arg or by setting walletcrosschain=1 in his bitcoin.conf file;
But without the blockchain, he won't be able to scan for transactions related to his wallet.