What reason could they have for firing her?
I know you said she's butted heads with people, but does she push back against the bullying or sit there and absorb it? Best advice would be to go to HR and put in formal complaints about all the people giving her shit. In theory, they're meant to be impartial and have your back in situations like this.
Next step would be a tribunal. I don't know too much about American employment law, but here's the first thing that came up when I googled it:
https://www.lexology.com/library/det...3-31769bc093dd. Basically the law should protect her right to work and not be discriminated against. Whether she's doing anything specific to 'provoke' this behaviour is irrelevant, it's still discrimination.
I don't care how close the person tormenting her is to the management, they won't be able to protect one another once a potential lawsuit raises its head. Even if your friend is going to leave there eventually, it's a matter of principle to give these pricks a rough ride on her way out of the door. Plus, regaining her dignity will do wonders for her self-confidence as and when she does decide to move on.