Chances are your friend owes more on the car than it's worth, if he bought it new with no or little down payment. Once it's repo'd they will sell it at auction for as much as they can get - most likely not as much as he owes. That money will be deducted from his loan amount, but he'll probably be facing repo fees, auction fees, etc that will be added to his total amount owed. Your friend will be responsible for the difference between his loan amount and fees and the amount they can sell it for at auction. Say he owes 20k. They repo the car & sell it at auction for 15k, and he has 1k in repo fees & such (totally pulling this all out of thin air). 20k + 1k = 21k total owed. 21k - 15k = 6k your friend is still responsible for paying.
Chances are the loan company will try to make post-repo payment arrangements with your friend for the balance. If that doesn't work out, they'll get a judgment against him for the 6k plus legal fees and it will sit on his credit report, making it very difficult for him to get another car loan for the next couple of years.