Hear the whole story before you pick a side. The first version is rarely the full one.

First accounts come from whoever got to you first, and they are shaped by what that person needs you to think. That is not necessarily dishonesty; people genuinely remember their own side more clearly. Choosing a side then commits you to defending a version you have not checked.

Delay the verdict and say so plainly, that you do not know enough yet. Ask the other person what happened, if you reasonably can, and ask what they wanted rather than who was wrong. Being the one who waits makes you the person both sides can still talk to.