The friend asked to mediate inherits an argument without inheriting either side’s full history.

Someone asked to mediate inherits a conflict without inheriting either person’s history of it. They get the current dispute and none of the years underneath, and they are expected to judge a moment that is actually the visible end of something long. Fairness with partial information is close to impossible.

If you are asked to step in, resist the role of judge and take the role of translator instead. Ask each side what they want the other to understand rather than who was right. And if you are pulling someone into your argument, be honest that you are handing them a fragment, since they will otherwise assume they have the whole thing.