I'm unclear where you learned that the employee had signed such a contract, especially if in that jurisdiction they are not bound by it. Did you ask to see copies of their past employment contract? I would feel zero obligation to mention this to my future employer if it doesn't apply in my jurisdiction.

It's like, if I had signed a non-contract stating I'm now a slave, I'd just ignore that contract. (What I'm saying applies to California.) Certain things just don't have any legal significance. (Such as indentured servitude.)