my son was playing golf with 3 friends, a car driving by was hit by a golf ball, the car drivver blamed the kids, who is liable

This is a very general question. Your son may be liable and you may be liable. His friends may be liable. Questions to be answered would include your son's age and that of his friends. Where was he playing golf. The golf course itself may have liability. If the car hit suffered and real damage and if anyone may have been hurt you should certainly notify your own auto insurance company as well as possibly your home owners insurance. In addition you may wish to place on notice the golf course.