How to get out of Upside Down Car Loan when already behind?

Need advice for a friend-I thought I would ask here since I myself have never been in this situation. He joined the army few years back-had NO credit @ the time. A car dealer on base took advantage of him (and many others),sold him a car, told him to come back since his credit didn’t approve him for the one he drove away with, and they put him in another car with a MIVE interest rate & he was promptly upside down in the car. For over two years, he never made one late payment-but when he discharged from the army-he had to go on unemployment till he recouped from back surgery. During that time he fell behind in payments & has been struggling ever since. He can’t get re-financed since he is already behind in payments, (the finance places said he can’t have made a late payment in past 6 months), & his credit score is low. He owes around 13k on the car, & its worth only $6k. So nobody will refinance until he stays current for 6 mnths-but he can’t get caught up & DONT want repo!