So called "friends" will act like they really like you, be friendly with you. And then goes out and make up lies to your girlfriend that you're seeing someone else and that he has been covering for you for the whole time. This makes your girlfriend seem confused and think you do not take your relationship seriously.
When we found out what that backstabbing prick did, we encircled him and beat the living sense out of him. Now he cannot talk and cause any harm to anyone.