Unfortunately, there is no clear cut answer to this question. Employers get the disability and leave laws wrong a lot, but the only way a lawyer can tell you if your father was wrongfully terminated is by consulting with him. A consultation on this would generally take half an hour to an hour to get the information necessary to render an opinion. In any event, I suggest that he talk to a lawyer as soon as he is able, because, as I said, employers get this area of law wrong a lot and he may have a case.
Gender, age, religious belief, disability, national origin, race etc. all are discriminatory reasons for which you cannot be legally fired and upon which you might base a wrongful termination suit. If you are an at-will employee, you can be fired for any reason or no reason, except unlawful discrimination. If you do not have an employment contract or union to represent you, your recourse is limited. Discrimination based on age, disability, gender, race, religious beliefs etc.... may give rise to a different answer.
You might find my Legal Guide helpful "Workplace Discrimination: A Basis for Wrongful Termination Claims"
You might find my Legal Guide helpful "How to Choose a Lawyer for you.”
You might find my Legal Guide helpful "What Do I Tell My Lawyer"?
No one can know what the record is in the case because online we cannot find out any details. If you have a discrimination theory about retaliatory discharge or hostile workplace conditions you'll need a lawyer. Check with a lawyer in your locale to discuss more of the details.
Good luck to you.
NOTE: This answer is made available by the out-of-state lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding.