The witness can decline if not subpoenaed--without a subpoena, it's just a "request," without legal force. But if the witness is subpoenaed, he or she will have to be deposed; if the witnesses refuses, he or she could be sanctioned (punished) by the court for "contempt" of court, and the defense could make a motion to ban the witness from testifying at trial, making his or her testimony unavailable to the plaintiff