The Juror Misconduct- Friend of the Co-Defendant's Family on my son's jury.

A jury of 12 (not of his peers) many, were several years older. One, with a daughter, was leaning NOT GUILTY. One was a friend of the Co-Defendant's family. In spite of the juror seeing her friends in the courtroom, she never divulged that she knew who they were. This information was uncovered by a post verdict investigation. Can a friend of the co-defendant's family truly be UNBIASED?