Assuming that the car is insured, the policy covers this car and any damage caused by this car, so 19 yr old guy can get his car repaired/reimbursed for. To get any fair recourse from the 'friend' your parents need to report that the vehicle was taken and driven without their permission to the gardai. The best advice I can give you to pass on to your parents is ring the gardai and their insurance company asap.
In the event that the vehicle was not insured - 19 yr old guy will be settled thru his own policy, with his insurers acting as the MIBI. He may well look for the excess from your parents but, if your parents have reported this matter to the gardai - then there is no reason why the 'friend' would not be pursued directly.
Was your brother or anyone else injured as a result of this?