Meh. If her son (and his friend) is smart and responsible, and have prepared a firm itinerary, its no big deal.
I could have easily managed it when I was 15-16
I went around a couple countries in Europe when I was 17.
A friend and his brother did a 2 week bicycle tour when he was 14. His brother was 16. His mom made him go to keep an eye on his older brother.
They later opened a bike shop.