The way you are describing the situation it definitely reads like something from a Better Call Saul episode. Technically the employee is still in a probationary period so you are free to do what you want really, and provided the supervisor didn’t actually assault the employee, you are probably within reason to terminate effective immediately, but make sure you have everything documented and conduct your investigation properly because I would hazard to say that this employee will definitely file a claim against you – provided everything is as you have said, I doubt they would win, but it will take up your time and energy.