[Sans-Culotte]

It may not have escalated to violence. He may have just left her somewhere. Or she may have stormed out of the relationship. Perhaps into the desert in the middle of the night. He might have reached his limit and drove off, calling her bluff. She may yet turn up in the company of new companions. Or her bones may be bleaching in the sun where she expired on her own. We may or may not ever know.

Regardless of how far on the crazy/hot matrix the girl could be, the boyfriend is guilty of something. Even if all the scenarios discussed are true, the boyfriend should have called her parents and told them "she ran of into the desert and I can't find her" (or whatever may have happened). The fact that he drove back alone and said nothing shows he did something and his parents are protecting him. It is not normal to not inform the girl's parents or the police of her disappearance.

[ArtDodger]

Right, you are. He may have made a bad decision and left her in the desert. Now, to avoid manslaughter through negligence, he is not cooperating with the police, on the advice of his lawyer. He may even have tried to cover up his mistake by faking the phone call, compounding his problems.
He may be just stupid or he may have whacked her, time may tell.