If your son was giving a citation (I cannot tell what code sections he was charged with), he has the right to a trial. If he feels there was no violation, he should demand that trial and try to win it (I assume this is an infraction, not a misdemeanor crime).
Then the questions - would the conversation between the officer and the lieutenant be admissible in evidence for that trial?
The answer is no. It's not relevant to the question of whether or not a violation existed. Either the violation existed or not It the vehicle did in fact violate the vehicle code (I assume that is what code we are talking about here), then he is guilty. If it did not, he is innocent.
If your son wants to win a trial on this sort of infraction, he should focus on gathering the evidence he will need to prove the vehicle was in compliance with the law.
Now, I supposed it is possible here to challenge the legality of the stop, based upon the idea that he was pulled over without probable cause because the officers knew him. Then, only after they stopped him illegally, did they find a reason to write a citation.
If those were the facts, a suppression motion would be the right way to request dismissal due to an illegal stop. Such a motion would argue that the police violated the 4th Amendment to the US Constitution prior to the stop.
However, if the officer did have a reasonable suspicion, based upon specific facts he observed, that the vehicle was in violation of a statue, the stop will be legal.
Does it matter that they decided not to give him a warning because they knew him and did not like him? No. That information will not help your son win the trial or get the citation dismissed.
It might be frustrating, but it will not help your son establish that he is innocent of the alleged violation. To win that trial, your son will have to show the vehicle complied with the law and that the officer is wrong.
(or perhaps proof of correction if it is a correctable offense).
It is not illegal discrimination to deny someone a warning because they know that person.
In general, illegal discrimination is defined as denying someone a right based upon their religion, race, skin color, sexual orientation, gender identity, or age (in some cases).
Getting a warning is not a right that can be illegally denied.
If they gave him a ticket that he was innocent of because he was Jewish, or stopped him illegally because he had a Muslim beard, then it would be illegal discrimination.
Hope that helps.
Follow up questions?