That is very illegal. Call the local sherrif's office, get a hold of a lawyer(at least for advice on state laws).
Retaining her property is also tantamount to larceny.
It will be a pain in the ass and require dedication and time to bring up charges, but I'm sure it can be done.
There is never any guarantee of legal punishment, but it is quite possible that she can get one or both parents with probation or even minimal jail time.
I'm not an expert, but have had a bit of experience with criminal and civil court..
I am in no way condoning revenge, just giving you the facts. Just curious, what was the fight over? You don't have to tell us if its private but I can't understand why in the world parents could do that to their kid.