I'm thinking of revealing to a prospective employer of my former employee that he shares pay information with coworkers, possibly costing him a job
How do you intend to prove it?
I am going to reveal the location of the information.
I can understand it can be wrong and against company rules to share private pay information with each other. You don't want to get involved in that though. You should keep it to yourself.
The company does not have a business in doing illegal things. I will warn them about the dangers of sharing work related information through a shared folder and corrupting the system.
Yes, I can see how it can be frustrating knowing that someone is doing something wrong, but you can't reveal something that could hurt someone's chances at a job. Let them find out for themselves what the person is like.