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. “
Have you ever had anyone save your life? Would you testify against them or go to jail for contempt?
And Jesus Christ is a convicted criminal...