These are two separate legal issues.
1. Can the underlying debt be voided
2. Can the decree of divorce be changed/voided (based on #1 or some other basis)
The underlying debt CAN be voided. Sure...if you can show that the underlying debt was not valid (since it was based on illegal activities) then you can void that debt. That is actually a fundamental principal of contract law (and debt is founded in contract law). Under contract law, agreements that relate to activity that that violates the law, or debts that come from activity that violates the law, can be voided. So the answer to #1 is easy. Yes. You can void the underlying debt (if you can show it was the product of an illegal activity)
But #2 is more complex.
Divorce decrees are difficult, and in some cases, impossible to modify. This is for legal and policy reasons. For good reason. If it was easy to do, then the court would be clobbered with requests to do just that.
In order to convince the court to change a decree of divorce you need one of two things
1. Both parties agree
2. Fraud on the court. If you can show fraud to the court (that one party lied to the court to get the change) that can be grounds to change the divorce decree.
This is complex since one of the parties is dead. But perhaps not impossible. To change the decree you would need to file a motion with the court (not the probate
court but the family court) and ask for the change based on the lies of the party that obtained the order for payment.
But all this said, I am not sure you need to modify the decree...you just need to get the executor to agree that the debt is not valid. If you can do that, they have the power to not pay it.
So you can go to the family court to try and change the decree...but it may be a better use of resources to simply focus on the fraud behind the order and see if you can get the executor to declare that the debt is not valid.