Dennis also got blamed by Darnell who said: kiddo you sold him that piece of **** you oughta be fking ashamed of yourself!
WTF?
Tell you what…When I send my ex-wife her money…you can lick the stamps.

The mother was insufferable.
She was an image conscious, controlling narcissistic bitch. Her husband was weak too. She thought she meant well, yet she couldn't deal with her own insecurities and entitlement issues, and passed the blame onto Dennis.
Arnie was free to make his own choices and whilst Christine was evil and took a hold of him because of his vulnerability, Christine was something his folks assisted in manifesting into their lives as well, due to how they smothered and pressured Arnie to be something they expected of him.
Norman! What did you put in my tea?

Exactly!
This irritated me, as well.
Dennis was Arnie's
friend
, not his mother. It wasn't his job to be Arnie's moral compass. The mother was domineering and insufferable. I get that he was 17 - a minor and still under her upbringing but clearly her constant nagging and incessant need for control was clearly toxic to the family dynamic. This, no doubt, had an impact on Arnie.

I never liked that she was so mad about it. She and Arnie's dad should have gone outside and looked at the car together.
I did sixty in five minutes once

Probably out of desperation. She sees her nice but nerdy son turn into a rebellious beep and has to blame someone.