How does one fight a lie?
Typically, a lie is the endorsement of falsehood.
Falsehoods are not intrinsically problematic. A falsehood is merely a statement in error. It is harmless, in and of itself. The problem is when a falsehood is mistaken for truth.
A lie's harm depends upon the severity of the misbelief. Which is to say, the significance of the truth, which the falsehood replaces.
Unfortunately, some statements are neither true nor false, until they are made, at which point, they become true. Many statements are simply opinions, or judgements. Objective reality is helpless against attitudes.
To declare someone an enemy can transform reality. The declaration is all it takes to make it true. Because enemies are defined by our opinions of others, not by any objective assemblage of facts.
Someone may have done nothing to harm me. Perhaps they even helped me. But I can declare them my enemy, and suddenly it is "true": we are at cross-purposes. I have stated my intention to harm them. Either literally, or by besmirching their reputation.
A relationship takes two to make it benevolent, but only one to make it antagonistic.
You cannot force someone to be your friend. You can only win them over by love, or seduction. If that fails, and they choose hostility, you must prepare to defend yourself as best you can. Perhaps by finding other friends.