YANBU. I would worry too. However, a person is no more likely to have a crash the day after she passes, than she is to crash after a month, or 6 months.
No. The more experience he gets, the better he will be. Especially moving from the 'driving lesson' situation of someone else watching, quiet conditions, focusing on the driving, to the 'social', drivign with friends and being in complete charge of the car situation. When you're learning, someone else is basically doing most of the navigation for you, for example! When you've JUST passed is the most dangerous time. Is that really not obvious?!
He will have passed his test just a day or two before. He will be new to driving without adult supervision. He'll likely be new to driving long distances. Instead of going out for a driving lesson, he'll be getting behind the wheel to come home tired, lairy, and quite possibly having had a couple of cans of beer or more ('go on you won't be over the limit on just two!'). Most of all, instead of being in a quiet calm car with an instructor or a parent, he'll be dealing with the inevitable distractions of the rest of the car full of mates laughing, screeching, messing about. As they do.
And that's exactly why there are cases and cases of this kind of jaunt ending in tragedy. There was one a few years ago where I used to live - country lane downhill - lost control and went into a wall. Four teenagers killed and one injured. All 17 and 16.
Just distracted, not used to driving while laughing with friends at the same time. No adult to say 'slow down coming round this bit'. No experience of driving solo really - passed the week before.
Tragic.
This is in no way demonising a young male driver. I'd be absolutely against ANY brand new driver taking responsibility for a carload and a day out and a drive back tired when they'd had practically NO solo driving experience and NO time getting used to longer drives or also having to watch road signs and navigate properly etc.
OP I really hope for your sake she doesn't go.