She should file a complaint with the NLRB, her state labor board, and hire an employment attorney to seek recourse. It’s the only way companies learn; you must apply economic pain and consequences.
https://www.nlrb.gov/about-nlrb/rights-we-protect/your-right...
https://www.seattletimes.com/explore/careers/can-you-get-fir...
Typically the company will want to know and be involved if you are acting as their representative.
Two different things going on here.
The NLRB and courts can sort it out, and at least the NLRB hasn’t been too fond of employer shenanigans and bad faith behavior as of late. An attorney quoted in the piece even mentions the person in question should contact an attorney, and she has nothing to lose and everything to gain by pursuing the employer through legal and regulatory mechanisms. What are they going to do, fire her more?
Sharing your salary is one thing.
Saying that you are an employee at X company and make X dollars is giving away both your salary and corporate information.
The key is not involving the company. Share your salary as much as you want, but don't go on social media and say 'I work for X' unless you are actually their representative.
The exception here is executives and maybe in some instances management, where they need to be careful what they say because anything they say may be construed as representing the company. But having on your social media that you work for X and having other posts where you talk about normal social media things? 100% fine as long as your don't say something along the lines of "I work for X and our official policy on Y is..." (i.e. claiming to represent them)