You can always tell a real friend: when you make a fool of yourself, he doesn’t feel you’ve done a permanent job.

Peter’s quip defines “real” friendship as a kind of moral and emotional generosity. Everyone occasionally embarrasses themselves; the difference is how others interpret that lapse. A true friend doesn’t treat a moment of foolishness as a fixed verdict on your character (“a permanent job”), but as a temporary misstep compatible with your better self. The line also implies trust: with genuine friends, you can be imperfect without fear that your mistake will be stored up and used to diminish you later. In Peter’s characteristic comic style, the joke sharpens a serious point about forgiveness, loyalty, and the refusal to reduce a person to their worst moment.