Prolly nothing new to say here, so I'll combine a bit of the above. First, pray for the rider and his family. If both of you are praying folk, you might spend some time praying together. As was mentioned, until you get solid information, you really know very little. The medevac doesn't tell us lots other than that he got to the hospital fast.
I agree with Heffay and our favorite uncle; the driver must do something wrong to rear end someone. I rear ended a car coming around an interesection, and the conditions were such that I don't know how I could have avoided it. (Complicated story -- delayed green of indefinite duration controlled by rookie officer, oncoming traffic, truck pulling out of blind alley, fresh rain.) Nevertheless, I take responsibility for it.
Of course he needs a friend now. As I believe EC stated, he'll tell you bits when he's ready. I've volunteered at a VA hospital. Although I read much military history, only once have I asked a vet about his service (
Chosin Marine Lt. Jos. Owen). When someone is ready to tell you, they'll tell you.
Down the road, when the shock has worn off as much as it can, I would ask him what concrete steps he planned to do to prevent future accident. Habits do die hard, and "I'm going to try harder" amounts to "I hope it doesn't happen again." To change, you need to make steps to change, and it could be something as simple has having all his passengers (off work, most likely) remind him when he is following too closely, or it could be a refresher course, etc.
