If the court sends the employee a subpoena, neither that employee nor their employer has a choice—they have to testify.
Not necessarily so. I was subpoenaed and my work informed me that I would be fired if I answered the subpoena. Seeing as how not answering could get me a bench warrant, I went to the judge and explained my “between a rock and a hard place” position. He told me to wait while he made a call. Came back and told me that apparently they had more power than him and he wouldn’t be forcing me to testify. Surprised me.