argument between coworkers

Refusal to comply could very much act against her own interests. I can write this script because it happens all too often. She's put in a grievance. The other person puts in a counter claim. She refuses to engage. The other person tells the manager "told you so, she's impossible" and the manager believes the other person.

In the end, spats like this are not going to get solved in a grievance procedure. Generally it results in absolutely everyone being unhappy with the outcome, making everything worse for the future, and the manager p****d off with both of them. And sorry to say this, but putting in a grievance over something that is said to you is confrontational. So saying that she won't participate in a discussion because it is confrontational is silly. If it is serious enough a comment, say something about it to the person. Don't hide behind grievances. Because you can't hide behind them and you simply ratchet everything up to a higher level. The other person knows about the grievance. They can't not know now. And unless they are very forgiving ( and a lot of people aren't) they will play nice, apologise, say they didn't mean it in the way it was received - and stab her in the back the first chance they get, and probably the second and third chance too.

Working with other people, you have to learn to take the rough with the smooth and reserve grievances for really serious stuff. Unless this was an offensively nasty comment, and discriminatory, it doesn't really hit the mark of serious.

But she started this. So it is down to her to see it through, and that means agreeing to meditation. Because refusing to agree will wrong foot her even more.