What do you do when you're falling for someone you shouldn't be falling for?

I realized that I'm falling for my best friend, who has a long distance girlfriend. The thing is that I know he is falling for me too. The problem comes in where he wants me to fight for him, and I don't want to do that. I don't want to be responsible for ending anyone's relationship. I'm scared because first and foremost I don't want to lose a friend. What should I do?