"To insist on one's duty and remain silent - that is the best answer to all slander." - George Washington
When you find out that someone is slandering you behind your back, you have an uncomfortable feeling, especially if that someone is your friend. Even though you are disappointed by this, you need to know that this is not about you, it is about them. They could do this from jealousy, envy, insecurity or because they feel threatened. Remember: if they talk behind your back, it just means you are ONE STEP AHEAD OF THEM!
Here's what to do when faced with it.
Don't be like them
Don't fight them, because then you would stoop to their level. Be better than that. Don't let them see that they hurt you. Hide your anger and disappointment from them. Don't give them the satisfaction of thinking they've won.
Let it go, don't react
Engaging in slander or directly confronting them will not stop them from doing so. They will continue to talk behind your back. So the best thing you can do is to ignore them.
Set clear boundaries
Don't start an argument with them if you don't have to. But don't pretend nothing happened either. Forgive them, but don't forget the incident and what they did to you. Your boundaries should be firm and clear. You must not allow anyone to play with them.
Be positive
Try to see the situation from their perspective. Why would someone act like that? Maybe they feel threatened, insecure or jealous. Try to see their reasons and understand them. Be nice to them. Be positive. Fight their negativity with your positivity and you will always win.