Virginity has nothing to do with this!! Sexual contact happened, so virginity is out the window and therefore a total waste of time to argue about.
Calling the girl a slut is a bit harsh since none of us were there when the sex occured. And in other words, if a guy has sex with a slut, what does that make the guy? A male slut, certainly not a good little boy!
If you were the parent if the girl, you would probably call the guys "sluts", so that statement is truly immature.
Point is, we don't know how the sex happened or how willing she was to have sex with 2 guys at the same time. She might not have screamed and fought to get the guys off her, because she was trapped in car with them, two guys against one girl maakes the girl lose, we just don;t know what happened. A couple of horny teenaged boys probably wouldn't notice or hear even a hesitation from a girl if they are about to "do her". I seriously doubt any guy would go home and tell their mommy they raped someone. Kids will tell their parents anything to get the parents to be on their side instead of getting punished.
Again, WE DON'T KNOW WHAT HAPPENED IN THE CAR!!!!
Get a lawyer for your son and hope for the best. Valuable lesson for the family to learn.