Welcome to Fluther.
It seems to me that the primary person responsible here is the driver of the car. But that won’t matter to the insurance company, who will expect to be made whole for their coverage of the injured party (and by “injured party” I mean the person who was rear-ended and suffered damage to the vehicle and may have also been injured personally; so one injured party could be the owner of the vehicle, and other injured parties – there could be more than one! – could be the driver and all passengers in the vehicle that was struck).
If your son’s girlfriend can’t cover the loss to the insurance company, then your son, the owner of the car, will be expected to step up and make them whole again – to pay them. The judgment to be expected is “your son and his girlfriend need to pay”, but this will be a civil matter, not criminal (at least based on what you’ve told us so far – if she was speeding, fleeing from a crime or evading a policeman who was attempting to stop her, for example, it could escalate into criminal court, too).
If they prove that they simply don’t have the cash to pay a civil judgment, then they will be expected to surrender assets – such as the vehicle itself, for starters – in settlement of the claim.