Performance can be subjective or objective. If you are evaluated based on objective criteria, try to keep any documents or evidence that would show you were a good performer. if the criteria by which you are judged is less clear, try to keep notes of your feelings of your performance and what is being said and done at work to make you feel the way you do. In most cases, to prevail in a claim for wrongful termination, you will have to prove you were a good performer.
This is a common complaint. Most likely the employee will be determined to be an "at will" employee, and as such, the employer can terminate the employee with cause, without cause for any reason or for no reason - just not for an illegal reason. I try to focus on what the employee believes the true reason to be for the termination. If the employee believes the employer had a good faith belief in theft, then I do not see a case.