They haven't done anything wrong AFAIK; they didn't sign off on an audit, and withdrew, which is the reality of many smaller accounting practices taking on unfamiliar clients.

Arthur Anderson knowingly fudged the numbers at Enron, and provided both accounting and consulting services, which was a big conflict of interest. The consulting arm came up with new type of revenue opportunities (energy contracts), and the accounting arm cooked the books to show that these were profitable.