I could be living at home right now but instead I choose to live on my own. I've lived in a variety of different places, with different people / arrangements. I've had to go back home before, but I only stayed for a short time until I found a place of my own. Most people I know who do well are not living with their parents. You have all of the financial considerations to keep in mind, but there's also the life experience aspect which is, I think, even more important.
I matured a lot more once I lived on my own and realized what it was really like. You have a false sense of security when you live at home with your parents. I saw it in siblings too, saw the change once they moved out, it's impossible to miss. I know some people that have stayed at home their whole lives and it really shows. They have zero ambition, they don't do anything with their lives, don't respect their parents, just overall pretty scummy. I know not everyone is like this obviously, but it's absolutely true that having to make it on your own wakes you up to reality and builds character for sure. It's a good thing.
The money side is another story, nobody forces you to pay $2000/mo in rent or buy an expensive house or take on huge debts. These are personal decisions. It's a shame that nobody seems to put much emphasis on personal financial responsibility. I was stupid with my money and made a lot of poor decisions, and it took me a long time to finally get my head out of my ass. But this is why I live in a very small apartment paying very little. I found a place that is within my means and within walking distance to every one of life's necessities and all the amenities one could want. I haven't had to make any compromises (except for it being a smaller space).
People justify their decisions because they "need" that huge place to live, well that's simply not always true. What you need is to make the right decision even if it's not what you want. I'm sorry, but living at home making $120k and buying expensive cars does sound shameful. If he can afford that he has no excuse not to be living on his own and building up his life and setting up his future. Blowing that money on expensive cars isn't going to help him do anything. Not that I personally care, I don't really mind what people do since it's none of my business - but you asked. So the answer is yep, it is shameful. If it was me i would feel shame. My parents already did enough for me, I don't need to take advantage of their kindness on top of what was already done for me.