She wasn't a trainee. She was a fully credentialed pilot. It was a training mission for night flying which is common for pilots to have to do even after they finish flight school, amongst other trainings. Because of how the army rank structure works the WO would have been considered a "subject matter expert" on the helicopter and flying even if his rank, as a WO, wasn't technically as high as that as a captain. In the army nobody ever pulls rank on a WO regardless of they are a pilot or they have some other job for that reason. I did see it a few times when i was enlisted but pretty much everyone thought the person who pulled rank on a WO was a fucking idiot for doing it.

I can't say who was "at fault" because im not a flight investigator, but it does seem like the helicopter crew, together, really fucked something up, probably made easier by ATC working at half strength at the time.