She hasn't worked a single hour and she's already bitching about them on social media? I'd say she did them a favor by not taking the job.
I don't see her bitching about them, she is declining the job and gives her reason why.
Writing them a letter is declining a job, posting the letter on social media is bitching.
Which she had every right to do.
I didn't say she was acting outside her rights, I just said she did them a favor by not taking the job. She seems like a troublemaker, she'd almost certainly make even more trouble for them if she took the job.