Sorry to hear of the accident; I truly hope your friend will have a complete and speedy recovery with minimal pain and complications.
I hope others will heed your words of wisdom; because of your age it may well mean more. When those of us who have passed the age of 40 say such things it's often thought we're just saying that because we're hopelessly out-of-touch old fogies.
I've had a heavy right foot all of my life, when I applied for admission to the bar I had to list and summarize every legal entanglement I'd ever had (including every moving violation). I'd be embarrassed to admit how many "extra pages" I had to attach to the application as a result of speeding violations.
All of that said, I will offer the following bits of learning from my own experience. Although it has been said that "speed kills", that is rarely true. Indeed, speed does not always kill. Moreover, speed -- in and of itself -- is not always unsafe. However, speeding on public roads always increases the odds against you, and speeding on roads for which you don't know the conditions that lie ahead is simply a challenge to the fates. Speeding may result in your being unable to avoid an accident, and in the event of an accident, the one sure truth is that speed will always increase the severity of the damage both to the car and it's occupants. And, lastly, even driving at or below the posted speed limit may be unsafe due to conditions, as you and your friend unfortunately discovered.
I hope you, your friend and those with whom you share this tale will soberly consider these things and live to be old fogies who are able to share such wisdom with others.
Take care and enjoy the ride,