Well, this is a criminal proceeding, so the standard is "beyond a reasonable doubt". If all the signatures in question are completely uniform and identical to those produced by the device, and Lay can demonstrate that subordinates had access to the device, he might get off (and rightly so under our system of laws, which prefers the abhomination of exhonorating the guilty to the abhomination of punishing the innocent because doing so restrains the threat to liberty posed by state power).
On the other hand, in the civil suits, where "preponderance of the evidence" is the standard, this won't help, and such ill-gotten gains as he still has should be taken from him and distributed to the victims of Enron's collective fraud.
It's Thomas Jefferson's fault.