She may very well be following her own managers lead, my 'training' in hospitality management was
"I need you to be the DM today, here's the keys" and the door didn't hit them in the arse on the way out because they were moving too fast! I had been there 3 months and was the most experienced member of
bar staff, in no way equipped to run the full hotel for 8 hours.
Thrown in at the deep end and left to get on with it with 101 issues to solve and no clue how to do it, and bollocked for getting it wrong.
You learn your management skills from those who manage you, and hospitality is not renowned for treating staff well!
You say you're used to being treated with more respect, which I get, but in general, hospitality staff are treated with little or no respect, the worst offenders being the upper management and owners/directors, so it becomes a culture.
I'm in no way saying that's right because it's the reason I left hospitality, but having experience in the area, I'd say it's more of a culture thing, and being 18 she's likely treating others how she herself has and is being treated, it's how some people think you get respect, and if they hold the hiring and firing rights, and you need the job.....
I think at 18 she's probably too young to have developed this style of management for herself because she'll lack the experience.