One of the most hurtful things ever is being stabbed in the back by a "friend." The pain of trusting someone only to find out that they have used you or are only interested in their own benefit is truly devastating.
Jesus understands. One of his disciples betrayed him by a kiss, receiving money to hand him over to people who would ultimately kill him. Another disciple named Peter denied even knowing him during his darkest trial.
Proverbs 27:6 says, "Faithful are the wounds of a friend, but the kisses of an enemy are deceitful." The Life Application Study Bible notes say, "Who would prefer a friend's wounds to an enemy's kisses? Anyone who considers the source. A friend who has your best interests at heart may have to give you unpleasant advice at times, but you know it is for your own good. An enemy, by contrast may whisper sweet words and happily send you on your way to ruin. We tend to hear what we want to hear, even if an enemy is the only one who will say it. A friend's advice, no matter how painful, is much more valuable."
So true friends are proven by their actions, not their flattering words. All people will disappoint us at times, like when Peter failed Jesus under pressure. But Peter repented (changed his ways) and history says that he eventually died for Jesus. Those actions prove he was a true friend.
What should we do when a friend stabs us in the back? Ephesians 6:12 says, "For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places."
Forgive them and take captive every thought. 2 Corinthians 10:5 "Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ." Don't have a shadow battle in your mind, you know those conversations where we say things to someone in our mind and they respond etc. If a root of bitterness tries to grow in your heart, keep chopping it down. Unforgiveness is just plain ugly.
Determine by their actions if they are a real friend of not, then ditch the fake friends. Find real friends who help you to become a better person and produce good fruit in your life.
Be filled up with the love that God has to offer you. Remember that even if the person who has harmed you claims to be representing God (like the religious leaders who killed Jesus), that doesn't mean God is actually behind their actions. God loves you. He is a true friend, proven by his actions. Take a look at his beautiful, nail pierced hands and remember that he understands how you feel. Let him comfort your broken heart.