Ask HN: How can I convince my friend not to buy that new car?

I also know a SWE who bought SLK despite all rational inputs from friends and relatives. Then in 3 years he changed it into VW Passat after he married and his wife was expecting a kid. That cost him some money as he needed to sell the car relatively quickly.

If your friend cannot be stopped getting a fast car, try to convince him at least to get Tesla which is faster than SLK and can be used as family car.

You probably can't.

The bottom line is that he appears to be an impulsive type of person.

You could try rational, logical arguments and they probably won't work. You need to understand his motivations about who he wants to impress. "Quality of life" might refer to who he wants to impress and what he thinks the new car will reflect upon his image. You might try to make him see that this is not the best path towards that goal.

Is it really going to be a "terrible mistake" or a learning experience that he will look back upon as another adventure in folly.

As a SWE he might pay it off quickly and having scratched that itch. It might be the motivation he needs to excel down the track or a passing fad. As a good friend you probably will be there to help him pick up the pieces if it comes to that.

Nothing short of excessive and counterproductive measures can stop him per day. Best I can see is trying to find a good enough car for far cheaper if things like the margins on autoloans and the absurd "times the same car sticker price would be paid for new in the pay off period".

Paying things with credit instead of savings is a /terrible/ mistake?