How to react when you come to know that someone you believe is back stabbing you?
Well, back-stabbing is a fact of life. Some people will disappoint you regardless of how well you behave. It seems inate in people to act stupidly at times.
In the Book of Proverbs we can read in chapter 10, verse 6: "Blessings [are] upon the head of the just: but violence covereth the mouth of the wicked." It could seem almost as though wickedness was a type of disease. Not a mental disease but a type of spiritual disease in the sense that wicked people DISregard good sound principles as honesty, loyalty, decency, amicability etc.
You will find that some can be "cured" when you tell them just how badly their attack affects you, but be prepared to disassociate yourself entirely if you aren't heard. You are much better off having only a few good friends in your Adress Book rather than having tons of people you can't trust.
So my advice is: stand up for yourself and be heard. If that doesn't work, the other person isn't worthy of being a friend of yours. Be friendly and you will attract the right type of people who SEE and recognize identical core values they also hold dear.