My son got into an accident and injured himself as well as his friend. How does liability work in this case?

One of the first factors that always come into play is determining the fault of the accident. The fact that your son hit the tree, the fault may be put on him, but then again there could be other factors such as a blown out tire which caused his vehicle to steer in that direction, an object in the road, or another vehicle that swerved into his lane. The case and evidence would have to be carefully evaluated and can become a lengthy process. In the mean time, it would be advisable to collect compensation for your son's medical bills through insurance. If your son’s friend is not insured, they may be able to legally pursue compensation from your son's insurance as he was the driver. The assistance of a Car Accident Attorney would be highly advisable as there are settlement factors to consider, fault accusations, and a defense against possbily being sued.