Is it against the law to record someone doing something illegal?

He does not know he has been recorded. He is the manager of a store where my sister-in-law works. He has been trying to make her quit her job so he can hire a girl friend. She called me today and taped him on my cell phone answering machine pulling his shinnanigans. I have it on my cell phone answering machine. You would not believe what he is saying.
Should I turn it into the corporate office or is it illegal?

Does she work in Illinois? If so, and if this site is correct, it would be illegal.
Twelve states require, under most circumstances, the consent of all parties to a conversation. Those jurisdictions are California, Connecticut, Florida, Illinois, Maryland, Massachusetts, Michigan, Montana, Nevada, New Hampshire, Pennsylvania and Washington. Be aware that you will sometimes hear these referred to inaccurately as "two-party consent" laws. If there are more than two people involved in the conversation, all must consent to the taping.

Regardless of the state, it is almost always illegal to record a conversation to which you are not a party, do not have consent to tape, and could not naturally overhear.

Does your sister-in-law know someone in human resources or does she have a contact with the district manager? This situation must be reported, whether your s-in-law is helped or not. It麓s worth the try!

I am pretty sure that's called evidence.

It depends on where the recording was done. In some states it is illegal to record a conversation unless all persons consent, and violating those statutes might result in severe civil or criminal penalties. This surreptious recording stuff is nothing to play around with. See a lawyer for advice specific to your situation.