choices. Hopefully she understands that. What you should be telling her about the accident and why she was hurt is this. The fact she was in a 300ZX might have kept her from being hurt more. Lesser cars have killed passengers in similar accidents. Really the root of the problem is not what car was she in. but rather why did SHE chose to get into a car w. someone that you (suspect) was under the influence of a substance? The problem here is not which car, but rather why would you place your life in the hands of someone who clearly doesn't seem to value their own. The lesson to learn here (and it's best to learn it young) is that we are not indestructable and the things (even our lives) that we take for granted can be gone in a second because of a bad decision or poor judgment.
As for course of action - get that guy out of YOUR life, and have HER take responsibility for what happened to HER that evening. From there figure out where you 2 are going as a couple or not.