There won't be an exit interview if she is laid off.

My bad. I caught that, but I didn't think much. Usually if one leaves a company voluntarily, the HR of the company will conduct an exit interview, which meant to avoid any legal issues in the future and to gain knowledge about reason of employee retention.

 Giving someone a hard time is very subjective. Laying off a person because the boss doesn't like the person is not a crime or misdemeanor. Previous performance review does not present a good evidence for "unfairness" either. She could argue that your wife’s performance declined. Your wife does not perform as well as she did before.

To sue on "unfairness", your wife will have to have some solid evidence, e.g. unfair work load, she is being treated differently from others, etc.