Has your defense witness suddenly taken the Fifth after receiving a warning from the prosecutor? You might want to find out exactly what the prosecutor said. Because if the prosecutor "assertive[ly]" threatened to make trouble for your witness if he so much as testified (whether or not he committed perjury), that warning just might be a Sixth Amendment violation.
Such was the case in United States v. Orozco, in which Chief District Judge Robinson (Kansas) vacated the defendant's convictions post-trial and dismissed his charges with prejudice: "While a limited warning of consequences for committing perjury is proper, a warning of consequences for simply taking the witness stand crosses the boundary line into improper witness interference." Dismissal with prejudice was necessary here because the prosecutor acted in bad faith and her interference prejudiced the defendant in several ways.