apparently her mate's two family cars were keyed too. i've seen the car now and whilst it isn't quite as bad as i was expecting from her phone call, it's still pretty horrific. 5 or 6 deep gouges 12 inches in length on the driver's door, 15+ on the boot up to a couple of feet in length and 3 or 4 on the bonnet about a foot long. thankfully it's only 3 panels and the others are all fine.
must have been a trouble maker at the party, so purely done out of spite towards the hosts and my sister's car just got caught in the crossfire.
a thank you to Hertfordshire Constabulary who actually sent out a unit to look at the cars and dust for finger prints. far better than Birmingham's Finest who actually told me they "didn't have time" to review the CCTV footage in the car park where my car was parked (in direct sight of a camera) when it was keyed a couple of years ago.