What todo if my wife’s ex-employer is saying she quit when in fact she got fired?

Whether she walked back to hand in her keys or not will not prove whether or not she was fired or quit: someone can quit and would still have to hand in her keys, after all--you don't get to keep your access to the business when you quit. So if there are multiple people who say that she quit, it may be very difficult to prove otherwise, especially since you can quit by actions as well as by works--such as by walking out during your job or shift.
As to the bonus: if she earned that bonus according to the terms of a written bonus agreement, by meeting the targets or goals in the agreement, they have to pay it to her unless something in the agreement itself gives them grounds to not pay: e.g. if the agreement states you must be employed when the bonus is paid to receive it. So if there is a written bonus agreement, your wife should review it and see if, under these circumstances, she is still entitled to it. If she is, then if she is not paid, she could sue for the money; she would sue based on "breach of contract," or the company violating the terms of the agreement.
But if there is no written agreement, then the bonus is most likely discretionary: one that the employer can choose to pay or not pay. If the bonus is discretionary--there is nothing in writing guarantying it--they can decide to not pay her.
As to vacation: in your state (KY), unusued vacation is payable when employment ends IF that is the company's policy or there is a contract requiring them to pay it. On the the other hand, companies may have a policy of not paying out vacation days, and if that was your wife's employer's policy, they would not have to pay her. She should look at any employee handbooks and also at past practice (did they pay other employees when employment ended?) to determine the policy. If the policy is that they should have paid her for her unused vacation time, she can sue for the money.