Is a friend sinful if he does not report the real killer and exonerates the innocent accused?

Bearing witness is a collective obligation (fard kifayah) if there are more witnesses than the prescribed number, and an individual obligation (fard ayn) if there are only enough witnesses to meet the prescribed number. It is not obligatory if only one witness or more come forward to testify, but not enough to meet the prescribed number. You must advise your friend to surrender himself and testify to what you saw, because concealing testimony is not among the forms of concealment that are required. However, if you are alone, you are not obligated to testify because your solitary testimony does not establish proof.