I am so sorry OP, you are obviously truly upset by this, and I can understand the temptation to justify his behaviour. I can imagine doing this myself, it just seems to fly in the face of what you know about someone.
It may well be that the other car was driving over the limit, because in theory if the boy (man) that you know felt it was completely clear, the fact that the other car suddenly appeared would imply that maybe they were going at great speed.
I can completely believe that your friend’s DS hadn’t done anything like this before because otherwise he would’ve known that it takes a lot longer to overtake than one might imagine.
I cannot clearly imagine why they didn’t both have time to do an emergency stop, it must’ve been very last minute.
Having said all that, your friend’s son made a huge mistake, and you have to acknowledge that it doesn’t matter WHY it happened, just that it did. He was on the wrong side of the road. He was not competent to get out of the situation/or was physically not able to get out of the situation. He should not have attempted this move.
It may have been an accident, as in “he didn’t mean it”, but it was still his fault.
I understand the argument with your DH. He is taking a very logical approach, you cannot help feeling emotional. I’d stop trying to make him see your view, which is pretty much untenable, and maybe find a quiet spot to feel how you feel. You won’t get much support from anyone, and I know it’s a shock, but it is one of the incredibly hard things about having adults as children.
They make terrible mistakes, they suffer the consequences. Sometimes they die. It’s awful.
Take care, and look after your close friend. Please don’t support her in the “it’s not fair” pit of sadness if he gets punished, it doesn’t help her in the long run. Or him. You can understand her sadness, and shock, but the idea that he is being hard done by is simply not true, and he will be a better man for facing the consequences, rather than having a “Mummy” who spends the rest of his life telling him he shouldn’t have to deal with the consequences like everyone else.
PS my closest friend at 16 was killed in a similar situation, hence my interest.