Okay, the whole show tends to consistently not be that realistic legally - it seems to be a CBS trademark - disregard for reality in favor of good showmanship, but this was a particularly egregious violation - the defenders should have moved to dismiss the charges against the kid, you can't convict someone on the basis of a charge unless you have a witness to accuse them. If the witness recants on the stand, it doesn't matter what impeachment evidence you have against the witness, the charge is dismissed. I'm a law student, I've been learning these things. And its complete nonsense that the case could have gone forward on the theory that the witness lied on the stand. Maybe they could later bring charges against the defendant for obstruction of justice - intimidating the witness through his attorney - but there are no charges for armed robbery if there is no one who testifies that they saw it happen. That's how our system works.
The show has great closing arguments but why can't they get the law right?