Someone did something, someone else took it badly, and now the group chat is split. This is where you bring the argument itself: the forgotten birthday, the plus-one nobody approved, the favour that was never returned, the joke that landed wrong. One situation, two sides, a verdict.
The jurors are ordinary people who have been the one in the wrong at least once and will say so. They judge what happened, not who writes better.
How cases are written here: in the first person, in plain words. Say what happened, in order, and what you want now; say what the other person wants in their own terms. Name the sides fairly ("Apologise" / "Let it go"), never "reasonable" against "unreasonable", and leave out anything you would not say to their face.