How hard is to be a parent, right? Even when your kids are old. Isn’t it?
I’m not a parent but I can figure out how many worries a dad or a mum can feel about if they are or aren’t doing well.

What happened to your kid had to be a tremendous trauma for you all.
It seems your wife is finding tougher than you to see what’s the best for your son just now.
If she’s diagnosed with bpd, there are many possibilities that she doesn’t want to punish your son or makes him learn a lesson ( that honestly mentioned, he already surely learnt the bad way) as it can seem. I would bet she’s full of fears. Many times bpd people don’t know how to use the language in an appropriate way. Much more than other kinds of people. We struggle to identifying feelings and facts and so we fail to express these with an understandable message. That is why the title of the known book “I hate you, don’t leave me” makes perfect sense for us but it’s so hard to decode by most of people.

I see so amazing that your son sounds so perfectly capable on go ahead, get into a car again and go on with his 24-y-o life. You both have the half-way done. It must be very hard for a dad or a mum sees how his kid get scared to face life bc of bad experiences.
Your wife has to go back to her 24-y-o and remember how she felt that age, how she thought, which her needs were. Surely, she has that age or younger a similar experience and she was punished maybe, how she felt, how she lived it. What she thought at that moment, would she have liked to be treat in a different way? Maybe, being the subject of more credit and trust in her abilities to learn and do things better...In the same sense, make her tell you what stop her from trusting your son’s future actions.

I do think she can be very open up to your words if you listen to her (I’m not saying you don’t do it), so, listen to her widely and actively so you can reach the point of her fears and make her understand that she has to renounces a few of her fears and quietness for the sake of your son.

Said that, it could be a good idea to search for a more insight input in other sub-forums: Here, I think you can also find them. Sub forums about parenting, relationships, people who loves people with personality disorders.

Good luck for your wife, you and of course, your son. He needs to be him and he will make mistakes. I’m glad he is out of fears after the trauma. It’s very positive while he has matured with the experience. You, your wife and him are the only one who know that for sure. :-)