Lily and Sirius aren’t friends, really. But the next time Marlene offers them Cauldron Cakes they may or may not glance at each other and laugh, like friends sharing an inside joke. When Evan Rosier walks around with a black eye on behalf of Lucius Malfoy, they may or may not share knowing smirks. She still can’t stand Potter, but Black...he may or may not be that insufferable after all.
Or, the one where Lily and Sirius are friends long before Evans and Potter are Lily and James. And that makes all the difference.