One idea would be to ask your JW friend to show you ANY dates in the Bible, perhaps starting with 1935 when heaven was closed by Rutherford. There is no Bible chronology per se, it ALL depends on historical records that exist in the Bible as kings and their reigns, along with tens of thousands of tablets and business records confirming them. There's a web site, SOS outreach if I remember correctly, that lists legitimate sources that establish 586/87 for the fall of Jerusalem, appropriately 607 sources. Ask your friend to offer one, just one, other than the WTS that says it was 607. Also include the WTS quote that basically says that yes, some secular historians and the scholars of christendom agree that 586/87 is the correct date, but that no one can deny that yet undiscovered evidence may be found that refutes all existing evidence. It's not a difficult arguement to show how flimsy and unreasonable it is to rely on totally unsupported claims because MAYBE in the future evidence MIGHT be found that would overturn TENS OF THOUSANDS of items of EXISTING proof. And the claim that SOME historians and scholars say 86/87, no, ALL historians and scholars. The simple fact is the WTS will ignore comprehensive and overwhelming proof in order to avoid admitting error, error that in this case debases the teaching of Christ's return, and far more importantly that shortly thereafter chose the WTS as his FDS.
So again, my basic suggestion is to show that there are NO dates in the Bible, so we rely on historians and scholars by necessity, who are unanimous. It is far more reasonable to see that the WTS cannot admit the 607 error because it removes their claim to FDS fame.