I forgot to mention that you are ALLOWED to be angry at your son's stupidity and this girl's behaviors. BUT, being angry does not give you permission to obsess about it, either, and that's what happens when we spend entirely too much time and energy upon something so negative and emotionally devastating to us.
WRITE OUT your anger on a piece of paper using a real pen - no techno-gadgets for this exercise! The act of writing down your anger on a piece of paper connects you with the reality of your feelings (anger, disappointment, disbelief, etc.) on a personal and physical level. By writing things out, you'll avoid saying or doing something that you'll seriously regret or something that will cause you to behave as foolishly as they are.