I've seen a couple posts/comment threads recently that talk about employers coming back to ex-employees asking for information. Frequently, the comments advise the OP to charge their former employer for said information, such as passwords or written documentation.
I'm not sure of others' experience, but every employment contract I've signed has clearly stipulated that all my files, notes, and other information (IP) are property of the company and I need to turn them over and destroy any copies I may have.
If I were to do as commenters are advising with that employment contract, I would likely be in some pretty hot water.
IANAL: Perhaps it would be better to say to a former employer, 'I don't have that direct information, but I'm happy to help you figure it out [for a fee]'. Once you do figure it out, they'll likely ask how you did it; that's a trade secret, and you're not going to tell them.
Repeat: I am not a lawyer. This is not legal advice. Just some food for thought that maybe you should talk to a real lawyer about.