Inform Police. God knows how many times we (fire service) have been out to a flat/house fire where some jealous boyfriend (admittedly usually scagheads) has torched the place through the letterbox.
When you ask the girl about it or if she thinks anyone might have done it she tells all about the ex and how she didnt report him cos she didnt want him to get in trouble...... and now she has all her belongings trashed and nowhere to live.
It is usually an escalation of events, from phone/texts... facebook!! You'd think there was no Delete button on that bloody thing... lurking outside home, vandalism and then more serious unless its knocked on the head.
I'd encourage her to make the call, esp with the threats of physical harm. Police having a word might be enough to stop it.
Of course the other way is to get her to tell her father. I dont know a dad on earth who wouldnt put a stop to this bolx in a second.