Several of the concerns raised in the article didn't related to frauds which the author felt should have been found, but with mis-statements of financial affair

Several of the concerns raised in the article didn't related to frauds which the author felt should have been found, but with mis-statements of financial affairs, which should theoretically be commented on by the auditor (the audit statment will generally confirm that the accounts present a "true and fair view" of the companies financial affairs). So if an organization is taking undue risks and not presenting them in the accounts, the auditor should make comment on that. In the example of HBOS, that could have been that they were understating the risk of bad debts and therefore had not provisioned sufficient reserves to address that risk. The suggestion of the article is that they don't do that due to a cosy relationship with their clients. It's hard to argue that there is an issue in the general case, auditors are paid by their customers, and therefore have an incentive not to rock the boat. The counterweight to that is meant to be the personal liability of the partners in question should there be a legal issue resulting from an audit, however I'm not aware of many suits against big-4 partners relating to collapses from the financial crash...