haha crash safety is a deep subject and cannot really be understood by anyone but a pro, but if u need proof there are a few crashed Zs on this site, i the 2thextreme or w.e his name was wrote one off and bought another. he was perfectly fine after an accident that wrote off his car.
as for logic (argument for ur gf), the 370z has a very rigid chassis. lightweight does not mean flimsy. if it did, all sports cars would shatter at the first corner. lots of structural braces, lots of safety equipment lots of safety stuff.
if she has any more complaints show her the fifth gear crash test with a small hatchback renault with a 5 star safety rating vs a big *** volvo station wagon. the results speak for themselves. i'll even link you. here u go
YouTube - Fifth Gear Crash-test Volvo 940 estate vs Renault Modus this proves that crash safety is not about who has the biggest car.