At the point where she did the “Heil Hitler” I would have called her over to a quiet corner and asked her directly, “What did you mean by THAT?” She might have thought it was a joke—at which point you could remind her that something that offensive should not even be done in jest, as it is deeply painful to all victims of ethnic violence. If, at that point, she showed remorse, you might have saved your friendship and taught her an essential lesson as well.