“If subpoenaed to give testimony in a trial then you don’t have a choice. If you refuse then that’s contempt and you go to jail for as long as the judge wants to keep you there. It makes no difference if it’s a gangland thug or a Marine NCO.”
Cant he plead the 5th?
You can plead the Fifth ONLY if your testimony might incriminate YOURSELF. There are only a few types of witnesses that can refuse to testify against their clients - lawyers, priests, doctors, spouses, and journalists (in some jurisdictions and ill-advisedly IMHO). There is not nor should there ever be a privilege because “it might get my friend in trouble. “
The 5th Amendment protects him from self incrimination. If he is asked to give evidence against someone else then the 5th doesn't apply.