I would be very very careful about your friends conduct here. The information you have been made aware of is not in the public domain - it has been disclosed to you as a defendant in criminal proceedings as part of the prosecutions duty to disclose to you any information which might assist the defence or undermine the prosecution. Hence the previous convictions of a prosecution witness are disclosed as this may undermine that witnesses credibility. The information was disclosed to you as part of the trial process - to ensure fairness to the defence. The witness has not given up any rights whatsoever.
It is a matter for you what you do with this information BUT I would anticipate that the Police woul;d take disciplinary action if it was made public and quite rightly in my view. The Police service has to employ people who are above reproach your friend clearly is not