Day 3 into recovery and have a small problem

So I have this friend of mine who I always adore. She is fiery strong in attitude and she has all the qualities that I want in my spouse. But she is in relationship with my best friend and that is how I became close to her. Everytime she talks to me she expects me to say something about him. And the bigger twist is that he is cheating on her and he thinks she is a bitch. So he is not serious. I got to know this when I tried convincing him that she is great. That is when I realised that she is a wonderfull person. I don't know if it is love or attraction but I feel bad for her everytime I talk to her. I told her that he is not serious, did not exactly say that he is cheating her. The real problem for me is everytime I talk to her I have a urge to fap thinking about her. I have this feeling as if to reassure her that she is the most lovable person. But I am actually feeling guilty after the fap. She is not thinking about me like that. Neither I can go behind the back of my best friend nor I can tell her that he is a big dick. What should I do? And also I shouldn't hurt myself more
Because I talked to her today on phone and the urge to fap suddenly returned. I almost relapsed but had enough willpower to stop it from happening. But I know it won't last long. So today my post is about the reason behind my urge. Will be great full to get an advice from other people. By the way she changed her WhatsApp dp and she is gorgeous but maybe if I fap thinking about her external beauty I will do great deal of harm to myself and it will be a big disrespect to her internal beauty. Right? And what if she comes to know about my second personality? I can't bring disgrace to such a wonderfull person even in my mind. I want to hear about other persons perspective because I am really confused

 

Thanks a lot for your valuable advice. I knew I was harming everybody. I just needed an other perspective. I got it now. I think maybe cutting her out or blocking her would deteriorate the problem. Right? Or is it better to do that?

 

I told him so many times. He is in another relationship.anyway Thanks for your valuable advice.it means a lot to me