If OP lives in an at-will state there’s really nothing they can do except retaliate if the company decides to press charges.

Even in an at-will state, claiming theft as a reason without proof still constitutes unlawful termination, as well as fraud.

You also need to thing in the long run. When this person applies for another job, the person could be denied the job because of the false accusation.

It depends on if they list theft as the reason for termination on their paperwork and then are able to provide evidence, or if that’s just what they told OP to get them to leave (then we’ve got an ethics and fraud problem) If they tell OP one thing but then put something else on paperwork, how could OP prove otherwise? Legitimate question.

They probably could, but is it really worth it for a job at McDonald's? Go find a job elsewhere. Not like there's a whole lot of potential left on the table at McDonald's.

In an at-will state I don't think they even need a reason, so why would they state theft

Because if they fire with a reason, they don't have to pay unemployment. If they fire you just to get rid of you, they will be paying unemployment to him.

You think OP has dozens of other job offers available? Safe to assume they were working at McD's because that was the best option available. So yeah, it IS a significant loss, to them, whether or not you think it's a good gig.

You can actually they need to supply the evidence in order to fire you for that issue

If OP is in an at-will state then they can’t sue for wrongful termination. The best chance they have at a lawsuit is if the company did lie about catching them stealing, OP could have a defamation suit on their hands. Especially if it keeps them from getting another job.