Tell Them One Thousand-and-One Times and Then Change Your Tactic
If you were watching that parent scream at his child, you might suggest he try another tactic rather than hostility and demands. You might suggest that another approach to his child might bear a more fruitful result. You might suggest that his frustration and rage, while perhaps completely justified, is not achieving the result he wants. And, sure, he might tell you to stick it up your ass but at least you tried.
...that when you spend time helping the truly broken—the ones who require more patience than seems reasonable—you walk away with two revelations: how much of yourself still works, and how vital the fragile thread of kindness is that holds people together.