It seems to me as though designating trustworthy individuals in our lives has become more challenging than ever before. There is more opportunity now than ever to abolish the trust you have for a person or a person has for you, therefore when someone you trust does something that hurts or offends you personally, it can be devastating, specifically when said someone is a close friend.
Friendships come and go throughout life, and I find this to be perfectly normal. People change and so do their values and beliefs, therefore differences can develop between individuals and their relationship diffuses. If you are fortunate enough to have people in your life who you refer to as true friends, hold on to those friendships. They are rare and are further incredibly valuable.
The concept of being two-faced is that something that can significantly impact relationships in a negative manner, whether it be a romantic relationship or a friendship. For example, if you are friends with someone who consistently talks about others when they are not present, but acts pleasant and kind towards them when they are face-to-face, it is arguable that this friend is two-faced. I have known two-faced individuals throughout my life, and when I recognize that someone they were talking poorly about is a main component in their social life, it prompts me to question what exactly this person is saying to others about me. If they gossip about who they believe to be their friends, it can be unsettling to comprehend that they may be saying negative things about you when you are not present.
If you are suspicious that someone in your life is two-faced, consider confronting them. If they openly admit to their actions, perhaps they will work to improve their behavior. If not, it is likely that they are not trustworthy, and your friendship may not be as valuable as you once thought it to be.