Damn there's some good sig material in here. A PROPERLY set up nitrous kit will not harm your motor so long as it's in good running condition. Modern kits are a far cry from the early days of nitrous. A quality wet kit or a dry shot with proper fuel management will be fine in moderation.
That said, your Lexus friend is an idiot. Don't let him touch your car.
That said, your Lexus friend is an idiot. Don't let him touch your car.