Sounds like orthostatic hypotension causing vasovagal syncope. She should wear compression socks and drink plenty of water. There are ways of mitigating and managing episodes. It's relatively common on planes.
I see some of them on Amazon and it looks like they're okay with TSA too, question though. If ( and I'm assuming this is an anxiety, not medical issue) your girlfriend is unconscious, would she possibly prefer to just remain unconscious rather than feel anxious about the flight? Or does she not like the fainting part either, i wouldn't know as I've never fainted so maybe it's unpleasant
If it's vasovagal syncope it's probably unpleasant. Can cause sweating, confusion, and a bunch of other problems because your brain isn't getting enough blood to function.
I bet a little prep and informed management of the situation can resolve it for her. It's important to keep blood pumping to get it out of lower extremities and into the brain. Walking in the aisles and leg exercises while seated are easy options.
Elevating the legs + lowering the head can also help. If she does pass out, laying her down and lifting her legs can quickly get blood to the brain. Alcohol and large meals can exacerbate it. Good luck!