Yes. Because the bank supplied him an agreement to review and then send back. But he REPLACED the agreement with his own fake agreement without the banks knowledge or permission.
The bank had no reason to expect the document to be replaced so even if they made a mistake by signing it they were tricked. We also don't know how professional the replacement document was but we can expect it was of very high quality and then it's clearly not reasonable to expect an ordinary employee to spot the fake.
Yes, I doubt that you can legally take a contract written by one party, make a few small changes, sign it and return it. He should have written the contract that he wanted or informed the bank that he head made changes to their version.