The defense tried to attack the witness' credibility or what his motivations were for sharing the letters. But it doesn't appear they even asked him direct questions like, "In your opinion, could anyone other than PF have manufactured those letters?" Or pressed him, under oath, relentlessly on specific details, "On what day and time and in what setting were you in when PF handed you letter 4? Where was PF standing in relation to you?" etc etc. They didn't go after any of his testimony at all, just tried to show he had incentive to give testimony that would be more favorable to the prosecution. Which hit a snag when it was told that in his other witness testimony case, he was testifying on behalf of the defendant.
If PF didn't write those letters, the defense should have been showing outrage and questioning the specifics and the handwriting and how the witness got this detailed information on people who were unknown in the press and public.
It would be extra amazing if the letter written on the back of the commissary receipt was from PF's own purchase with his account number.