Appropriate response when one friend is wronged by another who stepped out of line. “I disapprove of your recent actions. They seem to be completely out of character for you. If I believed you were that kind of person, I would have never become your friend in the first place. If you wish to continue our friendship, I demand that you immediately discontinue your unwelcome behavior and respect my boundaries as you have in the past. Please snap out of it. If you persist, I will have to stand against you to limit the damage you do in the future. I tell you this because I believe you might respond to constructive criticism. Otherwise, I would not be speaking with you at all. I hope we can get back to the way things were. The choice is yours.”
When Guy X is caught flirting with his best friend Y's date, Y responds, "Hey, fuck you, buddy. That's not cool."