if you can get your hands on a pressure tester, then do that. After about 10 minutes or so, if you don't see a puddle under the car, then pull ALL of the spark plugs and look into the cylinder with a flashlight. If it is a head gasket, you'll see coolant on top of one or more of the pistons. And tell your gf's dad that this is AUTO REPAIR, not some bullshit war with a middle eastern country that requires "more proof".