Grupo Mexico’s lead lawyer, Alan Stone of Milbank, Tweed, Hadley & McCloy, told me Monday that the company was “shocked” by Chancellor Leo Strine’s 106-page Oct. 14 ruling that the company owes $1.26 billion to the shareholders of Southern Peru Copper Company — the second-biggest shareholder derivative award in history.

It is a pretty shocking ruling. Strine found that Grupo Mexico, which at the time owned more than 50 percent of the shares of Southern Peru, basically forced the publicly-traded company to overpay, in stock, for Minera, Grupo’s privately-held Mexican mining company. In reaching that conclusion, the Chancellor managed to do three remarkable things: He rejected the judgment of Goldman Sachs, which advised Southern Peru on the deal; he questioned decisions by former Wachtell, Lipton, Rosen & Katz partner Harold Handelsman, who represented a minority owner of Southern Peru that was eager to cash out its stake; and he concluded that a special board committee of investment bankers conducted a tainted analysis. From the lead judge in a court that’s best known for making it almost impossible to prosecute a shareholder derivative suit successfully, this is a ruling every securities and corporate lawyer should read. (Plus, it’s written in Strine’s usual fluent, engaging style.)

Don’t cry too hard for Grupo Mexico (if you happen to be the sort of person who cries for corporate defendants found liable of breaching their duty to shareholders). Not only is it planning to appeal, according to Milbank’s Stone, but it now owns about 80 percent of the shares of Southern Peru, so even if it loses on appeal and has to pay up, it’s basically moving money from one pocket of Grupo Mexico’s fat wallet to another.

Nevertheless, for the sake of fairness, here’s Grupo’s comment: “The decision is not supported by the evidence presented at trial and is inconsistent with Delaware law,” Stone told me. “The court held the admittedly independent special committee … to an unrealistic and unachievable standard. Moreover, the court substituted its judgment not only for highly sophisticated board members but also for that of Goldman Sachs.”

So with all that gravitas weighing in support of the Southern Peru decision essentially to pay Grupo $3.1 billion in stock for Minera, how did plaintiffs’ lawyers at Prickett, Jones & Elliott and Kessler Topaz Meltzer & Check convince Chancellor Strine the deal was improper? (It wasn’t by working quickly. Strine quotes Vice-Chancellor Lamb, who handled the suit for several years after it was filed in 2004, criticizing the plaintiffs’ lawyers for failing to prosecute the case, then adds his own chastisement.)