What makes Tubal say that Antonio is undone?

As Act III, Scene I of William Shakespeare’s The Merchant of Venice comes to a close, the Jewish moneylender Shylock is discussing with his friend Tubal the situation involving the former’s business arrangement with Antonio, the titular character of Shakespeare’s play. As readers or viewers of Shakespeare’s play know, Antonio has stooped to borrowing money from Shylock in exchange for a pound of the merchant’s flesh should he be unable to repay the loan. In Act III, Salanio and Salarino are discussing their good friend Antonio’s misfortune, the merchant’s ship having run aground with its precious cargo lost. When the two men see Shylock approaching, they immediately disparage him, suggesting that he is the devil incarnate. When Shylock comes near, their tone moderates, but the underlying hostility and prejudices are still evident. Shylock is lamenting his daughter’s betrayal, and it is in this scene when he makes his impassioned plea for humanity (“Hath not a Jew eyes?”).
After Salanio and Salarino depart, Shylock encounter his Jewish friend Tubal. Shylock’s anguish regarding his daughter and the disappearance of much of his wealth, the daughter evidently responsible for the theft, is evident to Tubal who informs Shylock of the tragedy afflicting Antonio’s ship—a spell of good fortune for the moneylender desperate for revenge for a history of slights at the merchant’s hands. Tubal attempts to mollify Shylock, distraught by his daughter’s betrayal, by bringing up again the issue of Antonio’s misfortune. Hence, Tubal’s comment that “Antonio is certainly undone.” Tubal is stating what Shylock already now knows, that Antonio cannot repay his debt to Shylock because of the loss of the ship.

The words are spoken by Tubal to Shylock in Act III Scene i. What Tubal means is that Antonio has been financially ruined. One of his merchant ships has been wrecked on its way back from Tripolis. Tubal's had the news confirmed by some of the sailors who were fortunate enough to survive the shipwreck. He's also talked to some of Antonio's creditors, who say that he won't be able to avoid going bankrupt.
Shylock's overjoyed at the news. Now he's absolutely certain that Antonio won't be able to pay back the money he owes him. This means that Shylock will be able to enforce the bargain he made with Antonio and insist on having his pound of flesh. With his longed-for revenge seemingly at hand, a happy and excited Shylock instructs Tubal to fetch an officer of the law to have Antonio arrested.