Your gf is right and you are wrong.
Car windows block a maximum of 20% UV light.
I have the sunburns to prove it.
I even did a little unintentional experiment driving to NM from OK last year. The sun was always on the DRIVER'S side going westbound... My left arm was FRIED. Nothing else. This happened five times in a row.
Tinted the windows, repeated the process. No sunburn.
Clouds do not block UV either, so if you go out on a cloudy day you will still suffer UV damage to your skin.
I have done this experiement with a JETFIRE toy. The only areas that yellowed were those that were in direct or indirect contact with SUNLIGHT. AMBIENT sunlight was enough to turn the toy yellow. The areas hidden in shadow (parts of the wings that were folded, parts hidden by decals, insides of arms and legs, etc.) DID NOT YELLOW.
THE ONLY PARTS THAT YELLOWED WERE THOSE EXPOSED TO AMBIENT SUNLIGHT. THIS IN AN AREA OF HIGH HUMIDITY WITH MINIMAL AIR CONDITIONING USED.