From my experience the NDA for former employees are not worth the paper that they are written on.. If there is illegal activity then they should not use the NDA as an excuse.

Here is an example
California
In California (and some other states), there are some special circumstances relating to non-disclosure agreements and non-compete clauses. California's courts and legislature have signaled that they generally value an employee's mobility and entrepreneurship more highly than they do protectionist doctrine.