When friends break up, don’t use what you learned while you were close as ammo.

Closeness hands you information, and that information was given rather than earned. The insecurities, the family stuff, the thing somebody told you at two in the morning: all of it stays theirs, and using it when things turn sour converts a friendship into a weapon. That is a different kind of damage from an ordinary argument.

If something has gone wrong with a friend, decide in advance that the private material is off the table, especially in front of other people. Argue about what is happening between you now. The restraint costs one good line and saves you from something permanent.