Half of the teens killed in car accidents were not wearing seatbelts, per the CDC. Tell your teen to buckle up, regardless of who is driving.
We had a rule of a year of safe driving AND my personal judgment of the young driver before my kids would be allowed in the car. In my state, one doesn’t have to be a safe driver in practice in order to pass the road test, and there are some people who are never really good drivers. Thankfully, my oldest was a fantastic driver who had an amazing driver’s ed teacher, so I have always been confident in her driving. She’s going to be 30 this year and has never had an accident or a speeding ticket. So we bought her a car and she always has done most of the driving. My D2 hated to drive, so her situation was different, but her boyfriend (now husband) was a good driver, so after that year she was allowed to drive with him. But she had some friends who I never let her in the car with. My son refuses to get his license, which is a whole other issue. Many of his friends are, to put it delicately, not people who I would trust with his safety. I think you have to assess your situation, the friends, and the geography and make the best decision.
Are you sure that’s all he did? Didn’t give him the “shotgun talk” or anything? 
How did daughter take it?
Had to develop a family policy rather quickly after a mom texted me on a Monday to say that her son, who had just gotten his license on a Friday, was going to pick up my son to go to a sports practice on the other side of town that required going on two interstates through an urban area during rush hour. No. No highways with a new driver but I guess our policy allows for travel on city streets to local destinations- this hasn’t been happening but surely will this summer.
I told my children “no teen drivers”. I have relaxed it for a few drivers who are skilled, but the rule still stands. We live in a major city with crazy fast freeway drivers zipping in and out on the freeway that leads to our house in the suburbs. DD has one friend who has had her license from more than an year but is just a bad driver. She has had multiple accidents and DD knows to never ride with her. Take Uber. It is better to arrive alive.
Oh, yeah. I set rules that were sometimes inconvenient for my kids in this area. It meant I had to drive them more, which I did with no complaint. Also didn’t let them take other teens for longer than the state law allowed.
And even kids in the same family, taught by the same person, can be different types of drivers. I taught both of my girls how to drive (and they went to the same driving school to get certified to take their road test).
The oldest has a bit of a lead foot. Nothing too bad, but if the sign says 35 she will go 40.
My youngest is cautious, and won’t go a mile over 35.
They are both safe drivers. But different types of drivers.
The way some of my friends handled this issue (my D was the first of her friends to be licensed) was to inform ME that their children were not to drive with D for a period of time. I recall thinking I found it a little overbearing, however, once I was on notice, it was my responsibility to be honest, not their child’s. Then I thought ‘why not tell us?’ We, as the parents, owned the car and paid the insurance; D lived in our house, was 16 and was our responsibility.
Looking back, those friends of mine did the right thing by informing me and not putting the onus for honesty and responsibility on their kids; we all know, some 16 year old new drivers can’t be trusted, no matter how responsible and honest we think they are!
In California their friends couldn’t drive them until they’d had their licenses a year, so that never happened. I don’t think they were ever in a car with a teen driver. Almost impossible to get anywhere without going on a highway.
In Florida, they could ride with their friends from day 1. One friend had a new car 2 weeks before her 16th birthday. They drove with their friends, especially to school every day since it is so uncool to ride the bus.
Your rules about not driving with a driver until licensed for more than a year wouldn’t have done much good if my kids were the drivers. They each had a license long before they drove a lot. We had one car, and I drove it to work. My brother, on the other hand, had been driving since he was about 14 (on farms, motorcycles, snowmobiles) and was a great driver the day he got his license at 16.
One of my kids has had her license for 4 years and she’s not a good (or experienced) driver. She just hasn’t needed to drive that much. Many of our cars are manuals, and she can’t drive a manual.
@twoinanddone makes a good point that miles/experience is more important than the time. My D had a 40 minute commute to her HS and put nearly 30K miles on her car in 2 years. Much of that highway and small city driving, in all weather conditions. Now she has to drive through a major city to get to/from college. In HS, she had way more experience behind the wheel than most of her friends the same age.
We’ve got graduated license rules in NYS which apparently have reduced crashes by 80%. (Both curfew and restriction on passengers.) I was pretty bemused to see that a friend of my younger son’s had managed to flip her cute new car at a busy intersection with a light near the school where the speed limit is 30 miles an hour. She was lucky not to be hurt.
Then, you have to be careful about how you interpret the statistics. Suppose the risk of an accident is X% for a teen driver with no passengers and 1.5X% for a teen driver with one passenger. If you force two teens to drive separately as they’re not allowed to car pool, then the probability at least one of them will be in an accident is 2X% because you doubled the number of cars and teen drivers on the road.
" then the probability at least one of them will be in an accident is 2X% because you doubled the number of cars and teen drivers on the road."
Not how it works. 
^Close enough, The exact probability of at least one being in an accident is 1-(1-X)^(P+1), where P is the number of passengers who are now separate drivers. For one passenger, 2X% is a reasonable approximation, as the X^2 term is small enough to be ignored.
My kids did not ride with their friends while in high school. D1 drove herself after she got her license, until then we drove her every where. I remember there was one Fri afternoon when we drove her to her friend’s house, then drove her to a mall then back to her friend’s house. After a while, some parents got a wind of what we were doing and would ask us to drive their kids too.
D1 drove a mini cooper, so it wasn’t exactly a party car.
D2 had her own driver, so she never learned to drive.
Re: #31
In your example, the risk of at least one getting into a car crash is higher in the drive separately scenario. But in the carpool scenario, a car crash means that two are in a car crash. So the aggregate risk is higher in the carpool scenario.
Well, there was one kid I wouldn’t drive with since he’d speed into the driveway. My kids wouldn’t ride with him either. Ever. So no argument there. Great kid but no on the driving. Common sense.