>>testimony has already been given that GZ blood tests were negative for alcohol and drugs.<<
I believe you are wrong about that. Unless the cops arrested Z that night, they had no right to request tests from GZ, although I have not doubt that he would have agreed if they asked him. I believe the testimony showed that the cops had no suspicion that GZ was impaired.
I swear I heard testimony regarding GZ being tested for alcohol/drugs, but you seem pretty certain. Did the police testify to GZ being sober? I know this issue was discussed in court.