At the very least it makes sense to get the process laid out and draft any written portions with what's happened so far.

Figure out if there'll be a case number, or a response timeline, or what the likely possible outcomes might be

What measures you might take to shield against retaliation. Get your home copies of messages and data the company controls access to

Also see if coworker would be on boar with signing onto the complaint. Is it management in general or the one particular new PIC?

I'm encouraging her to speak to the union so we can back each other up. It's definitely this one individual. Though the store director being mostly laissez faire doesn't help.

If I had spoken to one of my baggers with the tone, volume, manor, and angry emotional invective she has when she loses her temper, I would expect to be written up or otherwise disciplined.

Update: had a 20 minute phone call with union rep and gave them a rundown of my concerns.